Add display list fallback reason histograms for 2D canvas

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 f0e568035490b8c1cb9eda522f715760bb82e773..e78e89fc0b34e5697f554cde75af241ee802a349 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3578,6 +3578,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Canvas.DisplayListFallbackReason"
+    enum="CanvasDisplayListFallbackReason">
+  <owner>junov@chromium.org</owner>
+  <summary>
+    The reasons why a canvas initially set to display list mode had to fall back
+    to direct rasterization mode.
+  </summary>
+</histogram>
+
+<histogram name="Canvas.GPUAccelerated2DCanvasDisableDeferralReason"
+    enum="CanvasGPUAccelerated2DCanvasDisableDeferralReason">
+  <owner>junov@chromium.org</owner>
+  <summary>
+    The reasons why a GPU accelerated canvas stope deferring it rendering

[Stephen White, 2016/01/21 21:02:31]

Nit: deferring it rendering -> deferring its rendering

[jwd, 2016/01/22 21:22:20]

stope -> stops? stopped?

+    operations.
+  </summary>
+</histogram>
+
 <histogram name="Canvas.HibernationEvents" enum="CanvasHibernationEvent">
   <owner>junov@chromium.org</owner>
   <summary>
@@ -57928,6 +57946,37 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="10" label="GPUAccelerated2DCanvasSurfaceCreationFailed"/>
 </enum>
 
+<enum name="CanvasDisplayListFallbackReason" type="int">
+  <int value="0" label="Unknown (Should not be seen in production)."/>
+  <int value="1" label="Canvas not cleared between consecutive frames."/>
+  <int value="2"
+      label="Canvas state stack too large (unballanced save/restore?)."/>

[Stephen White, 2016/01/21 21:02:31]

Nit: unballanced -> unbalanced

+  <int value="3" label="Direct pixel write."/>
+  <int value="4" label="Flush for initial clear."/>
+  <int value="5" label="Flush after drawing from a WebGL canvas."/>
+  <int value="6" label="Acquiring snapshot for getImageData()."/>
+  <int value="7" label="Acquiring snapshot for WebGL texture upload."/>
+  <int value="8"
+      label="Acquiring snapshot for direct painting of canvas contents."/>
+  <int value="9" label="Acquiring snapshot for toDataURL()"/>
+  <int value="10" label="Acquiring snapshot for toBlob()."/>
+  <int value="11" label="Acquiring snapshot for capturing to a stream."/>
+  <int value="12" label="Acquiring snapshot for drawImage() source."/>
+  <int value="13" label="Acquiring snapshot for createPattern()."/>
+  <int value="14" label="Expensive overdraw heuristic."/>
+  <int value="15" label="Darwing a texture backed pattern."/>

[Stephen White, 2016/01/21 21:02:31]

Nit: suggest hyphenating -> texture-backed

+  <int value="16" label="Drawing a video."/>
+  <int value="17" label="Drawing an animated 2D canvas."/>
+</enum>
+
+<enum name="CanvasGPUAccelerated2DCanvasDisableDeferralReason" type="int">
+  <int value="0" label="Unknown (Should not be seen in production)."/>
+  <int value="1" label="Canvas not cleared between consecutive frames."/>
+  <int value="2"
+      label="Canvas state stack too large (unballanced save/restore?)."/>

[Stephen White, 2016/01/21 21:02:31]

Nit: unballanced -> unbalanced

+  <int value="3" label="Direct pixel write."/>
+</enum>
+
 <enum name="CanvasHibernationEvent" type="int">
   <int value="0" label="HibernationScheduled"/>
   <int value="1"