scheduler: Add main thread gesture as a detected use case

components/scheduler/renderer/renderer_scheduler.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SCHEDULER_RENDERER_RENDERER_SCHEDULER_H_
 #define COMPONENTS_SCHEDULER_RENDERER_RENDERER_SCHEDULER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 27 matching lines @@
 class SCHEDULER_EXPORT RendererScheduler : public ChildScheduler {
  public:
   ~RendererScheduler() override;
   static std::unique_ptr<RendererScheduler> Create();
 
   // Returns the compositor task runner.
   virtual scoped_refptr<TaskQueue> CompositorTaskRunner() = 0;
 
   // Keep RendererScheduler::UseCaseToString in sync with this enum.
   enum class UseCase {
+    // No active use case detected.
     NONE,
+    // A continuous gesture (e.g., scroll, pinch) which is being driven by the
+    // compositor thread.
     COMPOSITOR_GESTURE,
+    // An unspecified touch gesture which is being handled by the main thread.
+    // Note that since we don't have a full view of the use case, we should be
+    // careful to prioritize all work equally.
+    MAIN_THREAD_CUSTOM_INPUT_HANDLING,
+    // A continuous gesture (e.g., scroll, pinch) which is being driven by the
+    // compositor thread but also observed by the main thread. An example is
+    // synchronized scrolling where a scroll listener on the main thread changes
+    // page layout based on the current scroll position.
+    SYNCHRONIZED_GESTURE,
+    // A gesture has recently started and we are about to run main thread touch
+    // listeners to find out the actual gesture type. To minimize touch latency,
+    // only input handling work should run in this state.
+    TOUCHSTART,
+    // The page is loading.
+    LOADING,
+    // A continuous gesture (e.g., scroll) which is being handled by the main
+    // thread.
     MAIN_THREAD_GESTURE,
-    SYNCHRONIZED_GESTURE,  // Both threads in the critical path.
-    TOUCHSTART,
-    LOADING,
     // Must be the last entry.
     USE_CASE_COUNT,
     FIRST_USE_CASE = NONE,
   };
   static const char* UseCaseToString(UseCase use_case);
 
   // Creates a WebThread implementation for the renderer main thread.
   virtual std::unique_ptr<blink::WebThread> CreateMainThread() = 0;
 
   // Returns the loading task runner.  This queue is intended for tasks related
@@ skipping 111 matching lines @@
       base::trace_event::BlameContext* blame_context) = 0;
 
  protected:
   RendererScheduler();
   DISALLOW_COPY_AND_ASSIGN(RendererScheduler);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_RENDERER_RENDERER_SCHEDULER_H_