Change logging of about:flags from actions to a histogram.

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 aca0658928cccb66e0936859f248eb27d7c10426..ecffef05d5ae1d8421ff953870ca4f926160db61 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -72,6 +72,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histograms>
 
+<histogram name="AboutFlags.Seen" enum="LoginCustomFlags">

[Ilya Sherman, 2016/08/29 21:19:59]

nit: WDYT of "FlagsAtStartup" rather than "Seen"?

[Ilya Sherman, 2016/08/29 21:19:59]

It looks like you're adding a new top-level category.  Any reason not to reuse
the "Login.CustomFlags" histogram, or at least log both sets of data to the same
histogram?

If we do introduce a new category, I'm wondering whether we could find a
slightly more general one -- e.g. "CommandLine.Flags.AtStartup".  Or, maybe it
makes sense to reuse the existing "Launch" or "Settings" categories?

[Alexei Svitkine (slow), 2016/08/29 21:50:58]

Good idea - renamed to Launch.FlagsAtStartup.

I plan to deprecate Login.CustomFlags in favor of this new histogram in a later
CL.

+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    Logs which Chrome flags from about:flags were active on start up.
+  </summary>
+</histogram>
+
 <histogram name="Accessibility.CrosAlwaysShowA11yMenu" enum="BooleanEnabled">
   <owner>dmazzoni@chromium.org</owner>
   <owner>kenjibaheux@google.com</owner>