Auto merge of #22923 - peterjoel:issue_8539_prefs_refactor, r=jdm

#8539 Config preferences backend restructure

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A procedural macro for generating static structures for use as the backend for config preferences, as well a mapping from string names to accessors.

Preferences can be accessed and updated via a map-like interface with `String` keys, and now also via a convenience macro: `get_pref!(path.to.pref)`. Various `serde`-compatible field attributes are also supported, including renaming the string keys. This could be useful when changing the backend structure without breaking existing usages.

I have added the choice to use `i64` as well as `f64` for storing numbers. As it turns out, none of the existing preferences used non-integer values. Setting a floating point value from a command-line argument requires a decimal point in order to parse correctly.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #8539

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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I have a few outstanding problems or questions:

1. I am unable to get rid of this warning:
    ```
    warning: unnecessary path disambiguator
       --> components/config/prefs.rs:130:51
        |
    130 |         accessor_type = crate::pref_util::Accessor::<Prefs, crate::pref_util::PrefValue>,
        |                                                   ^^ try removing `::`
    ```
    See: https://stackoverflow.com/questions/54710797/how-to-disable-unnecessary-path-disambiguator-warning
2. Several of the preferences in use were not represented in `prefs.json`. Some of these may be in error, but it is hard to tell. For example `js.offthread_compilation.enabled` vs `js.ion.offthread_compilation.enabled` could be different preferences or could be intended to have the same value.
3. Previously, several pieces of code provided default values when accessing a preference that may not be present. For example:
    ```Rust
    let DBL_CLICK_TIMEOUT = Duration::from_millis(
        PREFS
            .get("dom.document.dblclick_timeout")
            .as_u64()
            .unwrap_or(300),
    );
    ```
    Fallback values don't really make sense now and I've added these defaults to `prefs.json`. Does this sound right?
4. I have kept `PrefValue::Missing`, though it doesn't seem very useful any more. An error might be more appropriate now for an incorrect preference key. I've kept this partly because [`webdriver_server` uses it](https://github.com/servo/servo/blob/master/components/webdriver_server/lib.rs#L224).

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tests/unit/style/stylesheets.rs

@@ -9,7 +9,7 @@ use selectors::attr::*;
 use selectors::parser::*;
 use servo_arc::Arc;
 use servo_atoms::Atom;
-use servo_config::prefs::{PrefValue, PREFS};
+use servo_config::set_pref;
 use servo_url::ServoUrl;
 use std::borrow::ToOwned;
 use std::cell::RefCell;
@@ -339,7 +339,7 @@ impl ParseErrorReporter for TestingErrorReporter {
 
 #[test]
 fn test_report_error_stylesheet() {
-    PREFS.set("layout.viewport.enabled", PrefValue::Boolean(true));
+    set_pref!(layout.viewport.enabled, true);
     let css = r"
     div {
         background-color: red;

tests/unit/style/viewport.rs

@@ -6,7 +6,7 @@ use cssparser::{Parser, ParserInput};
 use euclid::TypedScale;
 use euclid::TypedSize2D;
 use servo_arc::Arc;
-use servo_config::prefs::{PrefValue, PREFS};
+use servo_config::set_pref;
 use servo_url::ServoUrl;
 use style::context::QuirksMode;
 use style::media_queries::{Device, MediaList, MediaType};
@@ -45,7 +45,7 @@ fn test_viewport_rule<F>(css: &str, device: &Device, callback: F)
 where
     F: Fn(&Vec<ViewportDescriptorDeclaration>, &str),
 {
-    PREFS.set("layout.viewport.enabled", PrefValue::Boolean(true));
+    set_pref!(layout.viewport.enabled, true);
     let stylesheet = stylesheet!(css, Author);
     let mut rule_count = 0;
     stylesheet.effective_viewport_rules(&device, &stylesheet.shared_lock.read(), |rule| {
@@ -445,7 +445,7 @@ fn cascading_within_viewport_rule() {
 
 #[test]
 fn multiple_stylesheets_cascading() {
-    PREFS.set("layout.viewport.enabled", PrefValue::Boolean(true));
+    set_pref!(layout.viewport.enabled, true);
     let device = Device::new(
         MediaType::screen(),
         TypedSize2D::new(800., 600.),

tests/wpt/metadata/css/css-flexbox/__dir__.ini

@@ -1,11 +1 @@
-prefs: ["layout.flex.enabled:true",
-        "layout.flex-flow.enabled:true",
-        "layout.flex-direction.enabled:true",
-        "layout.flex-wrap.enabled:true",
-        "layout.flex-grow.enabled:true",
-        "layout.flex-shrink.enabled:true",
-        "layout.justify-content.enabled:true",
-        "layout.align-items.enabled:true",
-        "layout.align-self.enabled:true",
-        "layout.align-content.enabled:true",
-        "layout.columns.enabled:true"]
+prefs: ["layout.columns.enabled:true"]

tests/wpt/mozilla/meta/css/__dir__.ini

@@ -1,2 +0,0 @@
-prefs: ["layout.flex.enabled:true",
-        "layout.flex-direction.enabled:true"]