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..7edf0c5f18d079b2338b207a99a1f7ed12daa5b3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -41552,6 +41552,26 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Sync.MemoryPressureWarningBeforeCleanShutdown" units="count">
+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times a user's sync service received a
+    MEMORY_PRESSURE_LEVEL_CRITICAL warning bbefore the sync service shut down

[Mark P, 2015/05/15 22:44:44]

nit: typo (bbefore)

[Gang Wu, 2015/05/15 22:53:48]

Done.

+    unleanly. The sync service emits this number when the next time the user's

[Mark P, 2015/05/15 22:44:44]

nit: typo (unleanly)
ditto below

Please proofread what you write.

[Mark P, 2015/05/15 22:44:44]

nit: omit "when"
ditto below

[Gang Wu, 2015/05/15 22:53:48]

Done.

[Gang Wu, 2015/05/15 22:53:49]

Done.

+    sync service is started. This count is per user.

[Mark P, 2015/05/15 22:44:44]

How about the following? Replace
is started. This count is per user.
with
is started, which will likely happen the next time the user's profile is opened
after a Chrome restart.  This count is emitted once per user/profile.  Things
like browser crashes that implicitly bring down all users' sync services will
cause unclean shutdown tags to appear on all open profiles, meaning that there
will be multiple emissions to this histogram as those profiles are re-opened.
(Is that all correct?)

ditto below

[Gang Wu, 2015/05/15 22:53:48]

Done.

+  </summary>
+</histogram>
+
+<histogram name="Sync.MemoryPressureWarningBeforeUncleanShutdown" units="count">
+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times a user's sync service received a
+    MEMORY_PRESSURE_LEVEL_CRITICAL warning before the sync service shut down
+    unleanly. The sync service emits this number when the next time the user's
+    sync service is started. This count is per user.
+  </summary>
+</histogram>
+
 <histogram name="Sync.NigoriMigrationState" enum="SyncNigoriMigrationState">
   <owner>zea@chromium.org</owner>
   <summary>Breakdown of sync's nigori node keystore migration state.</summary>