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

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 3c4bb306dfb03f97d06cd67a23ad734995ede71f..a771e15c13431902db3c7d2222da14c01ea0affc 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1640,12 +1640,19 @@ bool LayerTreeHostImpl::SwapBuffers(const LayerTreeHostImpl::FrameData& frame) {
   }
   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;