Enable WebRtcUseGpuMemoryBufferVideoFrame feature by default

Now that all the blocker issues(see bug below) are fixed, we can reenable this
feature.

Bug: 653200
Change-Id: I42c6bc582c4b7e4af770e093e46bbf12357c7753
Reviewed-on: https://chromium-review.googlesource.com/963642
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546751}

content/public/common/content_features.cc

@@ -519,7 +519,7 @@ const base::Feature kWebRtcUseEchoCanceller3{"WebRtcUseEchoCanceller3",
 
 // Use GpuMemoryBuffer backed VideoFrames in media streams.
 const base::Feature kWebRtcUseGpuMemoryBufferVideoFrames{
-    "WebRTC-UseGpuMemoryBufferVideoFrames", base::FEATURE_DISABLED_BY_DEFAULT};
+    "WebRTC-UseGpuMemoryBufferVideoFrames", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Controls whether the WebUSB API is enabled:
 // https://wicg.github.io/webusb