Add GPU.BlockStatusForClient3DAPIs histogram.

This data's been collected for some time, but due to an oversight
wasn't being visualized on the UMA dashboard.

Bug: 852884
Tbr: isherman@chromium.org
Change-Id: Id22a09ed788fa29172884d6f981e4acef52c4a19
Reviewed-on: https://chromium-review.googlesource.com/1114235
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570607}

tools/metrics/histograms/enums.xml

@@ -3396,6 +3396,13 @@ uploading your change for review.  These are checked by presubmit scripts.
              frozen."/>
 </enum>
 
+<enum name="BlockStatusForClient3DAPIs">
+  <summary>The reason WebGL / Pepper3D were blocked, or not.</summary>
+  <int value="0" label="BLOCK_STATUS_NOT_BLOCKED"/>
+  <int value="1" label="BLOCK_STATUS_SPECIFIC_DOMAIN_BLOCKED"/>
+  <int value="2" label="BLOCK_STATUS_ALL_DOMAINS_BLOCKED"/>
+</enum>
+
 <enum name="BluetoothAvailability">
   <int value="0" label="Unexpected error"/>
   <int value="1" label="Not available"/>

tools/metrics/histograms/histograms.xml

@@ -31767,6 +31767,16 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="GPU.BlockStatusForClient3DAPIs"
+    enum="BlockStatusForClient3DAPIs">
+  <owner>kbr@chromium.org</owner>
+  <summary>
+    Indicates whether WebGL / Pepper3D were blocked, and if so, for all domains,
+    or only a single one. This statistic is gathered every time an attempt is
+    made to create a 3D context.
+  </summary>
+</histogram>
+
 <histogram name="GPU.CheckFramebufferValidDuration" units="ms">
   <obsolete>
     Deprecated 01/2017.