Add UMA to measure feasibility of making unique names immutable

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 437b6c9db34da97dee857329f3718905cac176ac..31ff506008d4745c2e309260bf54042a14b404b1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -55751,6 +55751,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </details>
 </histogram>
 
+<histogram name="SessionRestore.SubFrameUniqueNameChangedBeforeFirstCommit"
+    enum="BooleanSuccess">

[Ilya Sherman, 2016/10/05 22:44:35]

Please provide an enum that's more specifically relevant to this histogram, e.g.
with labels "Kept old name" and "Assigned new name", or maybe just
Unchanged/Changed.

[dcheng, 2016/10/05 22:48:44]

Is there no generic boolean enum type? That's really what I want here, as the
behavior I'm measuring is already captured entirely in the UMA name.

[Ilya Sherman, 2016/10/06 19:08:30]

I agree that the name says a lot, but it's still easier to read the data if the
buckets have clear labels.  I guess it's not critical, but I'd definitely
encourage you to land a CL with more semantically contentful labels for the
buckets.

+  <owner>dcheng@chromium.org</owner>
+  <summary>
+    How often a subframe is assigned a new name between the initial empty
+    document and the first navigation.
+  </summary>
+</histogram>
+
 <histogram name="SessionRestore.TabActions" enum="SessionRestoreTabActions">
   <owner>chrisha@chromium.org</owner>
   <summary>