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 2727c9d64a7246568edcdf6dee523d467222f26e..e8e5ecfea48b27da5c575564ee30bfd21e03ff40 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2701,6 +2701,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ChromeOS.MemoryPressureHandleQuality.NonCriticalRatio">
+  <owner>xdai@chromium.org</owner>
+  <summary>
+    The fraction that the time system stays in non-critical memory pressure

[Mr4D (OOO till 08-26), 2015/02/01 10:16:34]

What about :

A by 1000% multiplied fraction of time spent in non critical vs. non critical +
critical time. This can be used to compare different memory pressure handling
schemes against each other whereas a higher number is better.

+    state. This is used to evaluate the service quality of the current memory
+    pressure handling scheme in ChromeOS.
+  </summary>
+</histogram>
+
 <histogram name="ChromeOS.PlatformVerification.Available"
     enum="BooleanAvailable">
   <owner>dkrahn@chromium.org</owner>