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..9d41ebae656ee600e21ff9bd12363c9819079065 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -41552,6 +41552,28 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Sync.MemoryPressureWarningBeforeCrash" units="count">
+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times each user's sync service received a

[Mark P, 2015/05/15 21:37:39]

nit: each -> a
ditto in other histogram

[Gang Wu, 2015/05/15 22:22:30]

Done.

+    MEMORY_PRESSURE_LEVEL_CRITICAL warning before sync service crashed. The sync

[Mark P, 2015/05/15 21:37:38]

nit: missing "the" before sync

[Gang Wu, 2015/05/15 22:22:30]

Done.

+    service emits this number on user's next startup. This counts are per user.

[Mark P, 2015/05/15 21:37:39]

nit (plural agreement): This counts are -> This count is
ditto in other histogram

[Mark P, 2015/05/15 21:37:39]

nit: on user's next startup -> the next time the user's profile is opened
ditto in other histogram

[Mark P, 2015/05/15 21:37:39]

You never answered this paragraph of my previous comment:
*In other words, is it possible that the sync service could crash for one user
but not another?

[Gang Wu, 2015/05/15 22:22:30]

yes, it is possible.

+    The sync service's crashes may cause by browser crashes, sync freezing or

[Mark P, 2015/05/15 21:37:39]

How is a crash a freeze?  Freeze makes me think of an infinite loop...
Or by crash do you simply mean unclean shutdown?  If that's what you mean,
please revise the histogram name and the description.

[Mark P, 2015/05/15 21:37:39]

nit: or -> ,

[Gang Wu, 2015/05/15 22:22:30]

removed this sentence.

+    etc.
+  </summary>
+</histogram>
+
+<histogram name="Sync.MemoryPressureWarningWithoutCrash" units="count">
+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times each user's the sync service received a

[Mark P, 2015/05/15 21:37:39]

nit: omit "the" before "sync"

[Gang Wu, 2015/05/15 22:22:30]

Done.

+    MEMORY_PRESSURE_LEVEL_CRITICAL warning before sync service shutdown cleanly.

[Mark P, 2015/05/15 21:37:39]

nit: add "the" before "sync"

[Gang Wu, 2015/05/15 22:22:30]

Done.

+    The sync service emits this number on user's next startup. This counts are
+    per user. The sync service's crashes may cause by browser crashes, sync
+    freezing or etc.

[Mark P, 2015/05/15 21:37:38]

This last sentence is not necessary or useful here.

[Gang Wu, 2015/05/15 22:22:30]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Sync.NigoriMigrationState" enum="SyncNigoriMigrationState">
   <owner>zea@chromium.org</owner>
   <summary>Breakdown of sync's nigori node keystore migration state.</summary>