Auto merge of #5608 - pgonda:cmdline-disable-style-sharing, r=Ms2ger

Let me know if I should fix anything, or how I could implement a test if needed.  I looked at some testing in servo/ports/command_line.rs but it did not seem to apply.

Thanks!

components/layout/css/matching.rs

@@ -32,6 +32,7 @@ use style::properties::{ComputedValues, cascade};
 use style::selector_matching::{Stylist, DeclarationBlock};
 use util::arc_ptr_eq;
 use util::cache::{LRUCache, SimpleHashCache};
+use util::opts;
 use util::smallvec::{SmallVec, SmallVec16};
 
 pub struct ApplicableDeclarations {
@@ -564,6 +565,9 @@ impl<'ln> MatchMethods for LayoutNode<'ln> {
                                         &mut StyleSharingCandidateCache,
                                       parent: Option<LayoutNode>)
                                       -> StyleSharingResult {
+        if opts::get().disable_share_style_cache {
+            return StyleSharingResult::CannotShare(false)
+        }
         if !self.is_element() {
             return StyleSharingResult::CannotShare(false)
         }

components/util/opts.rs

@@ -135,6 +135,9 @@ pub struct Opts {
 
     /// Whether MIME sniffing should be used
     pub sniff_mime_types: bool,
+
+    /// Whether Style Sharing Cache is used
+    pub disable_share_style_cache: bool,
 }
 
 fn print_usage(app: &str, opts: &[getopts::OptGroup]) {
@@ -162,6 +165,8 @@ pub fn print_debug_usage(app: &str)  {
     print_option("trace-layout", "Write layout trace to an external file for debugging.");
     print_option("validate-display-list-geometry",
                  "Display an error when display list geometry escapes overflow region.");
+    print_option("disable-share-style-cache",
+                 "Disable the style sharing cache.");
 
     println!("");
 }
@@ -216,6 +221,7 @@ pub fn default_opts() -> Opts {
         profile_tasks: false,
         resources_path: None,
         sniff_mime_types: false,
+        disable_share_style_cache: false,
     }
 }
 
@@ -377,6 +383,7 @@ pub fn from_cmdline_args(args: &[String]) -> bool {
         validate_display_list_geometry: debug_options.contains(&"validate-display-list-geometry"),
         resources_path: opt_match.opt_str("resources-path"),
         sniff_mime_types: opt_match.opt_present("sniff-mime-types"),
+        disable_share_style_cache: debug_options.contains(&"disable-share-style-cache"),
     };
 
     set_opts(opts);