Add UMA histograms for DirectComposition overlays.

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 55bbc66dd3f65704f415da0d687b580b6a3b6d6b..68221fe298b18e47dda04e10f2cdd47005f19c1f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21645,6 +21645,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="GPU.DirectComposition.CompositionMode"
+    enum="DxgiFramePresentationMode">
+  <owner>jbauman@chromium.org</owner>
+  <summary>
+    How DWM presented Chrome's DirectComposition layers to the screen.

[Ilya Sherman, 2017/03/29 05:16:42]

nit: What is DWM?  It might help to expand this acronym here.

+  </summary>
+</histogram>
+
+<histogram name="GPU.DirectComposition.OverlaysSupported"
+    enum="BooleanSupported">

[Ilya Sherman, 2017/03/29 05:16:42]

nit: Please use a custom enum here too (see below).

+  <owner>jbauman@chromium.org</owner>
+  <summary>True if Chrome will try to use DirectComposition overlays.</summary>
+</histogram>
+
+<histogram name="GPU.DirectComposition.OverlaysUsed" enum="BooleanUsage">

[Ilya Sherman, 2017/03/29 05:16:42]

nit: Please use a custom enum (just within this file), with labels like
"Overlay" and "No overlay".

+  <owner>jbauman@chromium.org</owner>
+  <summary>Whether or not a frame displays an overlay.</summary>
+</histogram>
+
 <histogram name="GPU.DoLinkProgramTime" units="ms">
   <owner>jmadill@chromium.org</owner>
   <summary>
@@ -88673,6 +88692,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="Unknown"/>
 </enum>
 
+<enum name="DxgiFramePresentationMode" type="int">
+  <int value="0">Composed</int>
+  <int value="1">Overlay</int>
+  <int value="2">None (Unknown)</int>
+  <int value="3">Composition Failure</int>

[Ilya Sherman, 2017/03/29 05:16:42]

nit: If you ever add additional enum entries, these will end up awkwardly in the
middle.  For this reason, I'd recommend actually moving these two exceptional
cases to the top of the enum.

+</enum>
+
 <enum name="EAPInnerProtocol" type="int">
   <int value="0" label="UNKNOWN"/>
   <int value="1" label="NONE"/>