HTMLVideoElement: add UKM for usage of prefixed fullscreen video methods.

webkitEnterFullscreen() and webkitEnterFullScreen() are rarely used and
could be removed. The exit counter parts are significantly more common.
The UKM may help us identify the top sites using these methods and drive
their usage down.

Bug: 745944
Change-Id: Iec937c4f851049355b49bb59bb4d4e32ab8445f9
Reviewed-on: https://chromium-review.googlesource.com/1163835Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580990}

chrome/browser/page_load_metrics/observers/use_counter/ukm_features.cc

@@ -50,6 +50,10 @@ bool IsAllowedUkmFeature(blink::mojom::WebFeature feature) {
           WebFeature::kPolymerV1Detected, WebFeature::kPolymerV2Detected,
           WebFeature::kFullscreenSecureOrigin,
           WebFeature::kFullscreenInsecureOrigin,
+          WebFeature::kPrefixedVideoEnterFullscreen,
+          WebFeature::kPrefixedVideoExitFullscreen,
+          WebFeature::kPrefixedVideoEnterFullScreen,
+          WebFeature::kPrefixedVideoExitFullScreen,
       }));
   return opt_in_features.count(feature);
 }