Update {virtual,override} to follow C++11 style in content/common/gpu/media

This change followed by the virtual methods and override/final section of
http://www.chromium.org/developers/coding-style/chromium-style-checker-errors.
Only one of {virtual,override,final} should be used for each declaration, since
override implies virtual.

BUG=417463

Review URL: https://codereview.chromium.org/906413003

Cr-Commit-Position: refs/heads/master@{#316894}
4 files changed