Implement "Save image as" for canvas.

Many users want to save the <canvas> as a image file. (like <img>)
As well as being useful, The feature is already supported in Firefox and IE.

Chromium side:
  https://codereview.chromium.org/263453007/

BUG=19277, 129710

Review URL: https://codereview.chromium.org/255563004

git-svn-id: svn://svn.chromium.org/blink/trunk@173505 bbb929c8-8fbe-4397-9dbb-9b2b20218538