New histogram to track overhead time spent around beforeunload event + fixes

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 dd89b52f6e84b34323b6c4b3884c4ed45bea44c8..11aafd379a965e778ee51c89e422e90859af77b1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -14008,6 +14008,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>The scheme of the URL for each main-frame navigation.</summary>
 </histogram>
 
+<histogram name="Navigation.OnBeforeUnloadOverheadTime" units="milliseconds">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Overhead time spent handling the OnBeforeUnload event from the browser
+    standpoint. In other words its total time the browser spent minus the time

[clamy, 2014/10/15 12:50:56]

I think this is a bit misleading: it reads as the overhead happens only in the
browser, when actually it doesn't. How about rephrasing it as:
More precisely, it is the total time between dispatch and acknowledgement of the
BeforeUnload event on the browser side, minus the actual time spent executing
the BeforeUnload handlers on the renderer side.

[carlosk, 2014/10/16 07:43:41]

Done.

+    spent solely by the call into javascript in the renderer.
+  </summary>
+</histogram>
+
 <histogram name="Navigation.RedirectChainSize" units="characters">
   <owner>haitaol@chromium.org</owner>
   <owner>donnd@chromium.org</owner>