customtabs: UMA histogram tracking whether clients call warmup().

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 df5c49d123550c50097d54c0d48ae94c41336e1e..8068398a165a128804e256e6c70076fe2b01240c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5137,6 +5137,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="CustomTabs.CalledWarmup" enum="CalledWarmup">

[pasko, 2015/09/03 18:02:20]

name suggestion:
CustomTabs.WarmupStateOnLaunch

+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    For Custom Tabs, records whether warmup() has been called, and if so,

[pasko, 2015/09/03 18:02:20]

please explain explicitly at what moment the histogram is recorded

+    whether the call could be matched with a session or not.
+  </summary>
+</histogram>
+
 <histogram name="CustomTabs.ClientAppId" enum="ClientAppId">
   <owner>yusufo@chromium.org</owner>
   <summary>
@@ -53879,6 +53887,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="4" label="CACHE_MISS"/>
 </enum>
 
+<enum name="CalledWarmup" type="int">
+  <int value="0" label="No Session, No Warmup"/>
+  <int value="1" label="No Session, Warmup"/>
+  <int value="2" label="Session, No Warmup, Called elsewhere"/>
+  <int value="3" label="Session, No Warmup, Not called"/>
+  <int value="4" label="Session, Warmup"/>
+</enum>
+
 <enum name="CanvasContextType" type="int">
   <int value="0" label="2d"/>
   <int value="1" label="webkit-3d"/>