Add high perf mode to VP8
Add high perf mode to VP8

R=marpan@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14709004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9c09e6ee2bc8af49af649a9ab107eb2201d47272

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