Add a <canvas>-tainting Setting. [2/3]

Add a <canvas>-tainting Setting. [2/3]

This setting will fall under the "potentially annoying security
features" flag; it certainly reduces the ability to use <canvas> for
fingerprinting, and some folks will want to turn it on, but it will
absolutely break significant subsets of the web.

Patch 1 - Blink:    https://codereview.chromium.org/828163005/
Patch 2 - Chromium: [THIS PATCH]
Patch 3 - Blink:    https://codereview.chromium.org/847993002/

BUG=448332
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/853463002

Cr-Commit-Position: refs/heads/master@{#311885}