Improved WebM performance on x86_64 Linux and Win64

Last week we landed patches from Makoto Kato to enable optimized assembly in libvpx on Win64. We also landed a similar patch from Tim Terriberry to enable optimized assembly in libvpx on Linux x86_64. This will improve decoding performance of WebM videos on these platforms. We're still being hit by scaling performance, particularly on high resolution and "full-screen" YouTube HTML5 experiment WebM videos. We're aware of this, and are working to improve it.