Break down end-to-end scroll update latency UMA metrics

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 1622 matching lines @@
 }
 
 bool LayerTreeHostImpl::SwapBuffers(const LayerTreeHostImpl::FrameData& frame) {
   ResetRequiresHighResToDraw();
   if (frame.has_no_damage) {
     active_tree()->BreakSwapPromises(SwapPromise::SWAP_FAILS);
     return false;
   }
   CompositorFrameMetadata metadata = MakeCompositorFrameMetadata();
   active_tree()->FinishSwapPromises(&metadata);
-  for (size_t i = 0; i < metadata.latency_info.size(); i++) {
+  for (auto& latency : metadata.latency_info) {
     TRACE_EVENT_FLOW_STEP0(
         "input,benchmark",
         "LatencyInfo.Flow",
-        TRACE_ID_DONT_MANGLE(metadata.latency_info[i].trace_id),
+        TRACE_ID_DONT_MANGLE(latency.trace_id),
         "SwapBuffers");
+    // Only add the latency component once for renderer swap, not the browser
+    // swap.
+    if (!latency.FindLatency(ui::INPUT_EVENT_LATENCY_RENDERER_SWAP_COMPONENT,

[jdduke (slow), 2015/02/11 23:22:02]

This seems like a common operation (adding a comonent if not found), and I keep
seeing it in various places. Maybe as a follow-up we could simplify all of the
callsites and add a helper routine?

[Yufeng Shen (Slow to review), 2015/02/12 02:59:02]

will do a follow up for this.

+                             0, nullptr)) {
+      latency.AddLatencyNumber(ui::INPUT_EVENT_LATENCY_RENDERER_SWAP_COMPONENT,
+                               0, 0);
+    }
   }
   renderer_->SwapBuffers(metadata);
   return true;
 }
 
 void LayerTreeHostImpl::WillBeginImplFrame(const BeginFrameArgs& args) {
   // Sample the frame time now. This time will be used for updating animations
   // when we draw.
   UpdateCurrentBeginFrameArgs(args);
   // Cache the begin impl frame interval
@@ skipping 1849 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc