Add UMA stats to get the quality of service of memory pressure handling scheme in ChromeOS.

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 2309a4abe4dc9104531674aa4cc9692facfb7595..7e23a5bd83bdce6439252e415ff9dfabaec27ede 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2701,6 +2701,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ChromeOS.MemoryPressureLevel"
+    enum="ChromeOSMemoryPressureLevel">
+  <owner>xdai@chromium.org</owner>
+  <summary>
+    The memory pressure level in ChromeOS, which is recorded periodically (once
+    per second). This is used to show the relative frequency of the system being
+    in a critical vs relaxed memory pressure state.
+  </summary>
+</histogram>
+
 <histogram name="ChromeOS.PlatformVerification.Available"
     enum="BooleanAvailable">
   <owner>dkrahn@chromium.org</owner>
@@ -43901,6 +43911,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="3" label="Reading"/>
 </enum>
 
+<enum name="ChromeOSMemoryPressureLevel" type="int">
+  <int value="0" label="No memory pressure"/>

[Ilya Sherman, 2015/02/04 23:13:16]

nit: Perhaps "Relaxed memory pressure" is a little clearer than "No memory
pressure"?  Up to you.

+  <int value="1" label="Medium memory pressure"/>
+  <int value="2" label="High Memory pressure"/>

[Ilya Sherman, 2015/02/04 23:13:16]

nit: "Memory" -> "memory" (lowercase "m")

+</enum>
+
 <enum name="ChromeOSPlatformVerificationResult" type="int">
   <summary>
     Possible results of a platform verification attempt. See