Add UseCounter for getFrameData.

This will allow us to determine how much magic window mode is used vs
presentation by looking at the ratio of this UseCounter vs presentation
UseCounter.

TBR=vollick@ for web_feature.mojom, which doesn't need owner review.
BUG=776086

Change-Id: I6e28433a00a8dbb9be3315d926d8f2b225be5b3e
Reviewed-on: https://chromium-review.googlesource.com/752079Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543541}

third_party/WebKit/Source/modules/vr/VRDisplay.idl

@@ -25,7 +25,7 @@ enum VREye {
 
     readonly attribute VRStageParameters stageParameters;
 
-    boolean getFrameData(VRFrameData frameData);
+    [MeasureAs=VRDisplayGetFrameData] boolean getFrameData(VRFrameData frameData);
 
     attribute double depthNear;
     attribute double depthFar;

third_party/WebKit/public/platform/web_feature.mojom

@@ -1879,6 +1879,7 @@ enum WebFeature {
   kCSSSelectorPseudoFocus = 2387,
   kCSSSelectorPseudoFocusVisible = 2388,
   kDistrustedLegacySymantecSubresource = 2389,
+  kVRDisplayGetFrameData = 2390,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

tools/metrics/histograms/enums.xml

@@ -17962,6 +17962,7 @@ Called by update_net_error_codes.py.-->
   <int value="2387" label="CSSSelectorPseudoFocus"/>
   <int value="2388" label="CSSSelectorPseudoFocusVisible"/>
   <int value="2389" label="DistrustedLegacySymantecSubresource"/>
+  <int value="2390" label="VRDisplayGetFrameData"/>
 </enum>
 
 <enum name="FeedbackSource">