Remove unused GC::writePixels() wrapper.

(only used in unit tests)

R=schenney@chromium.org,senorblanco@chromium.org

Review URL: https://codereview.chromium.org/657293004

git-svn-id: svn://svn.chromium.org/blink/trunk@183846 bbb929c8-8fbe-4397-9dbb-9b2b20218538