Annotate the races on static strings created by StringImpl::createStatic as benign.

This has been reviewed at https://codereview.chromium.org/48573005/ already.

TBR=abarth@chromium.org
BUG=223955

Review URL: https://codereview.chromium.org/50963002

git-svn-id: svn://svn.chromium.org/blink/trunk@160920 bbb929c8-8fbe-4397-9dbb-9b2b20218538