gpu: Add UMA statistic for RG texture support.

BUG=

Review URL: https://codereview.chromium.org/1068133002

Cr-Commit-Position: refs/heads/master@{#324310}

gpu/command_buffer/service/feature_info.cc

@@ -1090,6 +1090,7 @@ void FeatureInfo::InitializeFeatures() {
       texture_format_validators_[GL_RG_EXT].AddValue(GL_HALF_FLOAT_OES);
     }
   }
+  UMA_HISTOGRAM_BOOLEAN("GPU.TextureRG", feature_flags_.ext_texture_rg);
 
 #if !defined(OS_MACOSX)
   if (workarounds_.ignore_egl_sync_failures) {

tools/metrics/histograms/histograms.xml

@@ -11302,6 +11302,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="GPU.TextureRG" enum="BooleanAvailable">
+  <owner>reveman@chromium.org</owner>
+  <summary>
+    Whether RG texture support is available. Recorded each time a new context
+    group is initialized and extensions are detected.
+  </summary>
+</histogram>
+
 <histogram name="Hardware.Display.Count.OnChange">
   <owner>erikchen@chromium.org</owner>
   <summary>