Remove unused GraphicsContext::concat() return value.

Unused in Blink, going away in Skia.

R=schenney@chromium.org,reed@google.com

Review URL: https://codereview.chromium.org/196383019

git-svn-id: svn://svn.chromium.org/blink/trunk@169305 bbb929c8-8fbe-4397-9dbb-9b2b20218538