Removed unnecessary parameter from didActivateCompositor()

The parameter to didActivateCompositor() is meaningless. Hence, removed it. The other chromium side changes are here
https://codereview.chromium.org/137893025/

BUG=None

Review URL: https://codereview.chromium.org/138523003

git-svn-id: svn://svn.chromium.org/blink/trunk@170535 bbb929c8-8fbe-4397-9dbb-9b2b20218538