Add UMAs for GL context memory usage

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 88c426af8fd0ce84794be283d448b4dec117e10a..1da19770cce2f20b1de73c26da540afdfa90e751 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19777,6 +19777,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="GPU.ContextMemory" units="mb">
+  <owner>ericrk@chromium.org</owner>
+  <summary>The amount of memory used by a GL Context.</summary>
+</histogram>
+
 <histogram name="GPU.CreateBrowserCompositor" units="microseconds">
   <owner>vmiura@chromium.org</owner>
   <summary>
@@ -109981,6 +109986,20 @@ value.
   <affected-histogram name="GoogleSearch.AccessPoint"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="GPU.ContextType" separator=".">
+  <suffix name="WebGL" label="WebGL Context."/>
+  <suffix name="GLES" label="GLES Context."/>
+  <affected-histogram name="GPU.ContextMemory"/>
+</histogram_suffixes>
+
+<histogram_suffixes name="GPU.MemorySamplingTime" separator=".">
+  <affected-histogram name="GPU.ContextMemory.GLES"/>
+  <suffix name="Periodic" label="Sampled every 1s."/>
+  <suffix name="Shutdown" label="Sampled at shutdown."/>
+  <suffix name="Pressure" label="Sampled on CRITICAL memory pressure signal."/>
+  <affected-histogram name="GPU.ContextMemory.WebGL"/>

[Ken Russell (switch to Gerrit), 2016/12/20 20:28:03]

Should the two affected-histogram lines here go side-by-side for clarity?

[ericrk, 2016/12/20 21:19:08]

Yeah, no idea why I did it like that :/

+</histogram_suffixes>
+
 <histogram_suffixes name="GWSChromeJointExperiment">
   <suffix name="Experiment1"
       label="Only page loads that are a result of a navigation from a web