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

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index efdddecd19ca5885dc478f3a01ffe01ba4a1859b..43a6e03459fef70235b1592381ef3b51a5e35d97 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -7311,6 +7311,70 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Event.Latency.ScrollUpdateBrowserNotifiedToBeforeGpuSwap"

[Alexei Svitkine (slow), 2015/02/12 15:31:33]

Nit: I suggest adding another level of namespace to these to give them better
grouping on the dashboard, i.e.

Event.Latency.ScrollUpdate.BrowserNotifiedToBeforeGpuSwap

[Yufeng Shen (Slow to review), 2015/02/12 21:33:25]

Done.

+    units="microseconds">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Time between the browser receives the notification of a ScrollUpdate gesture
+    event induced renderer swap and GPU starts to swap.
+  </summary>
+</histogram>
+
+<histogram name="Event.Latency.ScrollUpdateGpuSwap" units="microseconds">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Time between gpu starts to swap a ScrollUpdate gesture event induced frame
+    and the swap finishes.
+  </summary>
+</histogram>
+
+<histogram name="Event.Latency.ScrollUpdateHandledImplToRendererSwap"

[Alexei Svitkine (slow), 2015/02/12 15:31:33]

For these pairs of histogram where the name only differs in "Impl" vs. "Main",
please use the histogram_suffixes construct to make the definitions more
concise. You can find an example in the file. Note: You can specify a custom
separator string - so e.g. the empty string or a dot, if you're not a fan of the
default _.

[Yufeng Shen (Slow to review), 2015/02/12 21:33:25]

Done.

+    units="microseconds">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Time between the ScrollUpdate gesture event is handled on main thread and
+    before renderer starts to swap.
+  </summary>
+</histogram>
+
+<histogram name="Event.Latency.ScrollUpdateHandledMainToRendererSwap"
+    units="microseconds">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Time between the ScrollUpdate gesture event is handled on impl thread and
+    before renderer starts to swap.
+  </summary>
+</histogram>
+
+<histogram name="Event.Latency.ScrollUpdateRendererSwapToBrowserNotified"
+    units="microseconds">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Time between the renderer starts to swap a frame induced by ScrollUpdate
+    gesture event and browser receives the swap notification.
+  </summary>
+</histogram>
+
+<histogram name="Event.Latency.ScrollUpdateTouchToHandledImpl"
+    units="microseconds">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Time between initial creation of a touch event and the generated
+    ScrollUpdate gesture event is handled on impl thread. If no swap was induced
+    by the ScrollUpdate gesture event, no recording is made.
+  </summary>
+</histogram>
+
+<histogram name="Event.Latency.ScrollUpdateTouchToHandledMain"
+    units="microseconds">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Time between initial creation of a touch event and the generated
+    ScrollUpdate gesture event is handled on main thread. If no swap was induced
+    by the ScrollUpdate gesture event, no recording is made.
+  </summary>
+</histogram>
+
 <histogram name="Event.Latency.TouchToFirstScrollUpdateSwap"
     units="microseconds">
   <owner>rbyers@chromium.org</owner>