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 056472e839e59af067ed64e026784f4494518478..f27b8bee32b992e378d545fa18948b72dd3a180c 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 current memory pressure level in ChromeOS, which is recorded

[Mr4D (OOO till 08-26), 2015/02/04 00:11:19]

I think you can get rid of the "current" here.

+    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>
@@ -43894,6 +43904,12 @@ 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="-1" label="Low memory pressure"/>

[Mr4D (OOO till 08-26), 2015/02/04 00:11:19]

Great that you added this enum!

This would start then from 0, .. and this entry in particular would be "no
memory pressure".

+  <int value="0" label="Medium memory pressure"/>
+  <int value="2" label="High Memory pressure"/>
+</enum>
+
 <enum name="ChromeOSPlatformVerificationResult" type="int">
   <summary>
     Possible results of a platform verification attempt. See