Implemented EXT_sRGB WebGL extension

Depends on https://codereview.chromium.org/638143003

BUG=386048

Review URL: https://codereview.chromium.org/636173005

git-svn-id: svn://svn.chromium.org/blink/trunk@184677 bbb929c8-8fbe-4397-9dbb-9b2b20218538