[cc/metrics] Reset the dropped-frame counter on navigation.

The dropped-frame counter needs to be reset after navigation to avoid
showing (or reporting) dropped-frame count from the previous page.

BUG=1115376

Change-Id: Id9fd54df5fcaf31b6d69e7e7d1190ac29581b6df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366613
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800284}

cc/metrics/dropped_frame_counter.cc

@@ -41,4 +41,11 @@ void DroppedFrameCounter::AddDroppedFrame() {
   ++total_dropped_;
 }
 
+void DroppedFrameCounter::Reset() {
+  total_frames_ = 0;
+  total_partial_ = 0;
+  total_dropped_ = 0;
+  ring_buffer_.Clear();
+}
+
 }  // namespace cc

cc/metrics/dropped_frame_counter.h

@@ -43,6 +43,8 @@ class DroppedFrameCounter {
   void AddPartialFrame();
   void AddDroppedFrame();
 
+  void Reset();
+
  private:
   RingBufferType ring_buffer_;
   size_t total_frames_ = 0;

cc/trees/layer_tree_host_impl.cc

@@ -4822,6 +4822,7 @@ void LayerTreeHostImpl::SetActiveURL(const GURL& url, ukm::SourceId source_id) {
     ukm_manager_->SetSourceId(source_id);
   }
   total_frame_counter_.Reset();
+  dropped_frame_counter_.Reset();
 }
 
 void LayerTreeHostImpl::AllocateLocalSurfaceId() {