Add more info to histograms.xml about omnibox 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 df0613de8423b8275326dd89e44e49417104393a..ee3db9d63c85171f2279bcd4d11dc51161a90867 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -42930,10 +42930,21 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="Omnibox.CharTypedToRepaintLatency" units="ms">
   <owner>asvitkine@chromium.org</owner>
+  <owner>mpearson@chromium.org</owner>
   <summary>
     Records the time taken between a keystroke being typed in the omnibox and
     the text being painted. If there are multiple keystrokes before a paint,
     logs the time since the earliest one.
+
+    This duration is composed of three parts: a) the time spent processing the
+    initial input event, b) the time spent for the repaint task to be scheduled
+    on the message loop and c) the time spent painting the Omnibox.
+
+    To diagnose a regression, check the following breakdown metrics:
+    Omnibox.QueryTime2 is a good proxy for (a) and Omnibox.PaintTime for (c).
+    (b) does not have a corresponding histogram but can be analyzed on the UMA
+    Task Profiler dashboard by looking at &quot;average queue time&quot; of the
+    task DisplayScheduler::ScheduleBeginFrameDeadline.
   </summary>
 </histogram>
 
@@ -43113,6 +43124,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="Omnibox.PaintTime" units="ms">
   <owner>asvitkine@chromium.org</owner>
+  <owner>mpearson@chromium.org</owner>
   <summary>
     Records the time to paint the omnibox contents. This is a subcomponent of
     Omnibox.CharTypedToRepaintLatency. Implemented on desktop platforms.