Added use counter for display:contents.

Bug: 657748
Change-Id: Ib2d298388c4a68a5992a5ef14e4fad28dee04c57
Reviewed-on: https://chromium-review.googlesource.com/823924
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523755}

third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.cpp

@@ -1656,7 +1656,10 @@ void CountKeywordOnlyPropertyUsage(CSSPropertyID property,
       }
       break;
     }
-
+    case CSSPropertyDisplay:
+      if (value_id == CSSValueContents)
+        context.Count(WebFeature::kCSSValueDisplayContents);
+      break;
     default:
       break;
   }

third_party/WebKit/public/platform/web_feature.mojom

@@ -1782,6 +1782,7 @@ enum WebFeature {
   kAudioWorkletGlobalScopeRegisterProcessor = 2262,
   kAudioWorkletNodeConstructor = 2263,
   kHTMLMediaElementEmptyLoadWithFutureData = 2264,
+  kCSSValueDisplayContents = 2265,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

tools/metrics/histograms/enums.xml

@@ -17238,6 +17238,7 @@ Called by update_net_error_codes.py.-->
   <int value="2262" label="AudioWorkletGlobalScopeRegisterProcessor"/>
   <int value="2263" label="AudioWorkletNodeConstructor"/>
   <int value="2264" label="HTMLMediaElementEmptyLoadWithFutureData"/>
+  <int value="2265" label="CSSValueDisplayContents"/>
 </enum>
 
 <enum name="FeedbackSource">