Add UMA/Telemetry stats for touch event latency

ui/base/latency_info.h

Index: ui/base/latency_info.h
diff --git a/ui/base/latency_info.h b/ui/base/latency_info.h
index 965d0d61f7f71c26f3a7504ab164ca9f42b02bde..2fe9433c882456cb3d33c0445ad2ba4077940f17 100644
--- a/ui/base/latency_info.h
+++ b/ui/base/latency_info.h
@@ -65,6 +65,11 @@ struct UI_EXPORT LatencyInfo {
 
   void Clear();
 
+  LatencyMap::const_iterator GetLatencyComponent(LatencyComponentType type,

[Rick Byers, 2013/06/26 21:42:10]

Rather than return an iterator, may be easier to use as:
bool GetLatencyComponent(LatencyComponentType type, int64 id, LatencyComponent&
output) ??

[Yufeng Shen (Slow to review), 2013/06/26 23:02:26]

Done.

+                                                 int64 id) const {
+    return latency_components.find(std::make_pair(type, id));
+  }
+
   LatencyMap latency_components;
 
   // This represents the final time that a frame is displayed it.