Fixed a performance regression with the upload-canvas-2d-to-texture test

Overhead of creating new extension helper objects on the fly was far
more than I had originally anticipated.

TBR=kbr@chromium.org
BUG=341814

Review URL: https://codereview.chromium.org/138383008

git-svn-id: svn://svn.chromium.org/blink/trunk@166749 bbb929c8-8fbe-4397-9dbb-9b2b20218538