DevTools: introduce "Track CSS Usage" experiment

third_party/WebKit/Source/devtools/front_end/timeline/TimelineController.js

Index: third_party/WebKit/Source/devtools/front_end/timeline/TimelineController.js
diff --git a/third_party/WebKit/Source/devtools/front_end/timeline/TimelineController.js b/third_party/WebKit/Source/devtools/front_end/timeline/TimelineController.js
index f075760e19d973c338122f44eb4ce567ed5c3ce4..5363aa95295f0c7473c1189af1c749b1d9d3dd23 100644
--- a/third_party/WebKit/Source/devtools/front_end/timeline/TimelineController.js
+++ b/third_party/WebKit/Source/devtools/front_end/timeline/TimelineController.js
@@ -18,6 +18,10 @@ WebInspector.TimelineController = class {
     this._tracingModel = tracingModel;
     this._targets = [];
     WebInspector.targetManager.observeTargets(this);
+
+    if (Runtime.experiments.isEnabled('timelineRuleUsageRecording')) {
+      this._markUnusedCSS = WebInspector.settings.createSetting('timelineMarkUnusedCSS', false);
+    }
   }
 
   /**
@@ -74,7 +78,10 @@ WebInspector.TimelineController = class {
     tracingStoppedPromises.push(new Promise(resolve => this._tracingCompleteCallback = resolve));
     tracingStoppedPromises.push(this._stopProfilingOnAllTargets());
     this._target.tracingManager.stop();
-    tracingStoppedPromises.push(WebInspector.targetManager.resumeAllTargets());
+
+    if (!Runtime.experiments.isEnabled('timelineRuleUsageRecording') || !this._markUnusedCSS.get())
+      tracingStoppedPromises.push(WebInspector.targetManager.resumeAllTargets());
+
     Promise.all(tracingStoppedPromises).then(() => this._allSourcesFinished());
 
     this._delegate.loadingStarted();
@@ -160,7 +167,9 @@ WebInspector.TimelineController = class {
    * @param {function(?string)=} callback
    */
   _startRecordingWithCategories(categories, enableJSSampling, callback) {
-    WebInspector.targetManager.suspendAllTargets();
+    if (!Runtime.experiments.isEnabled('timelineRuleUsageRecording') || !this._markUnusedCSS.get())
+      WebInspector.targetManager.suspendAllTargets();
+
     var profilingStartedPromise = enableJSSampling && !Runtime.experiments.isEnabled('timelineTracingJSProfile') ?
         this._startProfilingOnAllTargets() :
         Promise.resolve();