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..7127e48a8a6cdd8d727e339390ca35fa60dbf479 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -41552,6 +41552,20 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Sync.MemoryPressureWarningBeforeCrash">

[Mark P, 2015/05/15 17:46:09]

units=? or enum=?
ditto below

[Gang Wu, 2015/05/15 18:20:37]

Done.

+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    The MEMORY_PRESSURE_LEVEL_CRITICAL sync service received before crash.

[Mark P, 2015/05/15 17:46:09]

All histograms should clearly state when they are emitted.  Is this emitted on a
later startup after a crash?  Or later opening of a syncing profile that was
open during a crash?
ditto below

[Mark P, 2015/05/15 17:46:09]

Are you talking about a full browser crash here or something narrower, like the
sync service failing?

Also, depending on your answers to the other questions, if these are
stability-related histograms, you may want to consider marking them as such in
the code (UMA_STABILITY_...).  This has certain implications.

[Gang Wu, 2015/05/15 18:20:37]

Done.

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

[stanisc, 2015/05/15 20:31:04]

without should start with capital W

+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    The MEMORY_PRESSURE_LEVEL_CRITICAL sync service received without crash.
+  </summary>
+</histogram>
+
 <histogram name="Sync.NigoriMigrationState" enum="SyncNigoriMigrationState">
   <owner>zea@chromium.org</owner>
   <summary>Breakdown of sync's nigori node keystore migration state.</summary>