[DevTools] Resources: add "copy image as data url" menu item.

It is often convenient to grab image as data url.

BUG=321132

Review URL: https://codereview.chromium.org/88103002

git-svn-id: svn://svn.chromium.org/blink/trunk@162691 bbb929c8-8fbe-4397-9dbb-9b2b20218538