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..f935adcd7cb7edec4f08cb8698aeee3b08d08467 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="count">
+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times sync service received a
+    MEMORY_PRESSURE_LEVEL_CRITICAL warning before sync service crashed. The sync
+    service will emit this number(even is 0)  during next startup.

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

nit: bad spacing
(here an below)

nit: is -> if it is
(that's what you meant, right?  I don't know what "even is 0" means.)

+  </summary>
+</histogram>
+
+<histogram name="Sync.MemoryPressureWarningwithoutCrash" units="count">
+  <owner>gangwu@chromium.org</owner>
+  <summary>
+    Counts the number of times sync service received a
+    MEMORY_PRESSURE_LEVEL_CRITICAL warning before sync service shutdown cleanly.
+    The sync service will emit this number(even is 0) 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>