Split SpellCheckPanel off SpellCheckProvider

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 22d3a66c2235d6fc4e31ca793faf991648ea670c..ef56811f87c0b9499f69960581b70a4c708da941 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -67831,6 +67831,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SpellCheck.api.showUI" enum="BooleanEnabled">

[Ilya Sherman, 2017/04/05 21:43:54]

nit: (Just within this file) please use an enum with more custom-tailored
labels, like "Native OS platform UI" vs. "Chrome UI" (or whatever labels are
appropriate).

[Ilya Sherman, 2017/04/05 21:43:54]

nit: I have some quibbles with this metric name, but they're subsumed by my
comment below.  Mostly noting this as a reminder to myself, in case you convince
me that this metric's semantics make sense as-is.

+  <owner>groby@chromium.org</owner>
+  <owner>rlp@chromium.org</owner>
+  <summary>
+    The number of times the platform spelling UI was enabled/disabled.

[Ilya Sherman, 2017/04/05 21:43:54]

What is the intended use of this histogram?  This sort of metric tends to be
notoriously hard to use, because it biases toward users who flip the state more
frequently.  A user who chooses a state once and then never modifies it again
will be almost invisible.

I'd recommend instead sampling this state periodically.  For example, you might
want to record whether or not the platform UI is enabled each time that a
spelling suggestion is offered.

[Xiaocheng, 2017/04/05 22:17:48]

Thanks for the idea.

This patch is for fixing crbug.com/638361. This metric, being irrelevant to the
bug fix, is moved from spellcheck_provider.cc to spellcheck_panel.cc as is. I
think I should make sure in this patch that the description added here matches
exactly what is shown in the UMA dashboard now, without introducing any change.

A better design of this metric needs more discussion and is out of the scope of
this patch. I filed crbug.com/708791 for it.

[Ilya Sherman, 2017/04/05 22:54:07]

I see.  Is there a corresponding CL to remove the metric definition from
google3?

+  </summary>
+</histogram>
+
 <histogram name="SpellCheck.SpellingService.Enabled" enum="BooleanEnabled">
   <owner>groby@chromium.org</owner>
   <owner>rlp@chromium.org</owner>