Compilation fix

'OVERRIDE' keyword happened to go thorough CQ but should be 'override'
now.

This was introduced at r184100:
https://codereview.chromium.org/614413002

NOTRY=true
TBR=amikhaylova@google.com

Review URL: https://codereview.chromium.org/670103002

git-svn-id: svn://svn.chromium.org/blink/trunk@184148 bbb929c8-8fbe-4397-9dbb-9b2b20218538