Add UMA data for memory pressure warning received for sync

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 b13b369b4f61b5c71b0575e4c7a6cbc9620bfcd9..9154c788805f4bdbde16f612a29aa276ac462f49 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -41552,6 +41552,24 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Sync.MemoryPressureWarningBeforeCrash" units="Times">

[Mark P, 2015/05/15 19:18:23]

units="count"
I think would be clearer.  "Times" has some association with time.

[Gang Wu, 2015/05/15 20:05:56]

Done.

+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times sync service received

[Mark P, 2015/05/15 19:18:22]

nit:times sync service received -> times the sync service received a
(i.e., add "the" and "a")

[Gang Wu, 2015/05/15 20:05:56]

Done.

[Mark P, 2015/05/15 20:15:39]

nit: Not done; you forgot to add the "the".  sync service -> the sync service

[Gang Wu, 2015/05/15 21:13:47]

This is really Done :-).

+    MEMORY_PRESSURE_LEVEL_CRITICAL warning before sync service crashed. The sync

[Mark P, 2015/05/15 19:18:23]

How does the sync service crash?  How is this different than a normal browser
crash?  If the browser crashes, does that count as a "sync service crash"?

[Gang Wu, 2015/05/15 20:05:56]

Sync could crash for several reasons. 
Browser crash will cause sync to crash. and also, sync could crash(or stopped
unnormally, like freezing) without browser crashes. that is one of the reason we
want to this UMA, we want to know how many crashes from memory pressure.

So browser crashes will count into sync service crashes.

[Mark P, 2015/05/15 20:15:39]

Please revise the histogram descriptions to explain this.  Explaining to me in
the code review is not as useful for future people who may read it this
description.

+    service will emit this number during next startup.

[Mark P, 2015/05/15 19:18:22]

I notice you're storing this number in a preference store.  Is this per-user or
global?  (I'm not familiar with prefs to be able to tell.)  i.e., if I start up
chrome using a different profile after a crash, do we emit something?  Or is
this only emitted on the next open of the profile that was part of the sync
server crash, not necessarily the next startup?

[Mark P, 2015/05/15 19:18:23]

If this number is 0, it'll be emitted, right?  Please say so explicitly.

[Gang Wu, 2015/05/15 20:05:56]

Yes, even it is 0, the number will be emitted as well. added into comment.

[Gang Wu, 2015/05/15 20:05:56]

the Preference file, which used here, is per-user. There is a global preference
file, but did not use here. So every time user start up, sync will check this
user's preference.  

[Mark P, 2015/05/15 20:15:39]

Then when this emission happens depends on what user starts up chrome next. 
Please revise the description; saying "The sync service will emit this number
during next startup" is wrong.

Also explain what happens if multiple profiles are open when the sync service
crash.  Are sync service crashes per profile or global?  Would this count be
emitted for each profile on the next time it starts or only one profile on the
next time it starts?  What about if the browser crashes?  Is the answer about
what gets emitted the same?

+  </summary>
+</histogram>
+
+<histogram name="Sync.MemoryPressureWarningwithoutCrash" units="Times">

[Mark P, 2015/05/15 19:18:23]

Many of the above comments apply to this histogram description too.

[Gang Wu, 2015/05/15 20:05:56]

Done.

+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times sync service received
+    MEMORY_PRESSURE_LEVEL_CRITICAL warning before sync service shutdown clearly.

[Mark P, 2015/05/15 19:18:23]

nit: clearly -> cleanly

[Gang Wu, 2015/05/15 20:05:56]

Done.

+    The sync service will emit this number during next startup.
+  </summary>
+</histogram>
+
 <histogram name="Sync.NigoriMigrationState" enum="SyncNigoriMigrationState">
   <owner>zea@chromium.org</owner>
   <summary>Breakdown of sync's nigori node keystore migration state.</summary>