[Tab Metrics] Added TabManager.SessionRestore.SwitchToTab UMA.

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 f73b10dbb3381b6725685c9c6344d73bd295928a..f225a244c5dac3d61f6626d405b875a12b8dfc07 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -77597,6 +77597,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="TabManager.SessionRestore.SwitchToTab" enum="TabLoadingState">
+  <owner>fmeawad@chromium.org</owner>
+  <summary>
+    Number of times that a user switches to a tab during a session restore,

[Mark P, 2017/06/29 18:25:51]

I'd rather rephrase this.  It's not recording the "number of times". The
aggregate (sum) is a number.  Here's my attempt:

The loading state of a tab at the time a user switched this tab during session
restore.

[shaseley, 2017/06/29 19:45:16]

Done.

+    broken down by tab loading state.

[Mark P, 2017/06/29 18:25:51]

Please add information about:
* When I start Chrome and it begin restoring a session, is this initial tab that
gets shown first (not explicitly selected, but select nonetheless) recorded?
* If I bring a window to the front of a different Chrome window during session
restore, is that recorded?  Or is it not recorded because I haven't actually
switched tabs?
* Roughly, when is session restore considered over?

[shaseley, 2017/06/29 19:45:16]

Done, hopefully the text here is much clearer now.

[Mark P, 2017/06/29 19:51:30]

Much better now, thanks!

+  </summary>
+</histogram>
+
 <histogram name="Tabs.CountAtResume" units="tabs">
   <owner>lliabraa@chromium.org</owner>
   <summary>