style: Replace plugin behavior with a basic fallback

The browser currently only enables plugin behavior for Flash and our internal test plugins.  This patch replaces support for those plugins with a simple fallback that shows a transparent region where the plugin would have been.  It removes the file system search(es) for the plugin dynamic libraries and short-circuits the logic to determine if plugins should do something special -- all implementations now behave the same in the presence of plugin elements.

The new behavior is:
1. If the <object> or <embed> element lists a type of something other than "x-shockwave-flash" or "x-test" then the behavior is unchanged.  This means that non-plugin types behave properly and unknown types (for example, typos) are also unaffected (they reduce to 0x0 elements).
2. If the <object> element has an HTML fallback in the DOM (see spec for <object> elements) then the fallback is always shown.
3. Otherwise, the element is shown as a transparent region with the size specified in attributes.

Differential Revision: https://phabricator.services.mozilla.com/D95902

components/style/element_state.rs

@@ -144,6 +144,8 @@ bitflags! {
         const IN_MOZINERT_STATE = 1 << 54;
         /// State for the topmost dialog element in top layer
         const IN_TOPMOST_MODAL_DIALOG_STATE = 1 << 55;
+        /// Non-standard & undocumented.
+        const IN_HANDLER_NOPLUGINS = 1 << 56;
     }
 }
 

components/style/gecko/non_ts_pseudo_class_list.rs

@@ -72,6 +72,8 @@ macro_rules! apply_non_ts_list {
                 ("-moz-handler-disabled", MozHandlerDisabled, IN_HANDLER_DISABLED_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS_AND_CHROME),
                 ("-moz-handler-blocked", MozHandlerBlocked, IN_HANDLER_BLOCKED_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS_AND_CHROME),
                 ("-moz-handler-crashed", MozHandlerCrashed, IN_HANDLER_CRASHED_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS_AND_CHROME),
+                ("-moz-handler-noplugins", MozHandlerNoPlugins, IN_HANDLER_NOPLUGINS, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS_AND_CHROME),
+
                 ("-moz-math-increment-script-level", MozMathIncrementScriptLevel, IN_INCREMENT_SCRIPT_LEVEL_STATE, _),
 
                 ("required", Required, IN_REQUIRED_STATE, _),

components/style/gecko/wrapper.rs

@@ -2046,6 +2046,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::MozHandlerClickToPlay |
             NonTSPseudoClass::MozHandlerVulnerableUpdatable |
             NonTSPseudoClass::MozHandlerVulnerableNoUpdate |
+            NonTSPseudoClass::MozHandlerNoPlugins |
             NonTSPseudoClass::MozMathIncrementScriptLevel |
             NonTSPseudoClass::InRange |
             NonTSPseudoClass::OutOfRange |