Ship image-rendering: pixelated to stable.

This is effectively a revert of: 
https://codereview.chromium.org/604683003
Which was itself a revert of the first attempt to ship pixelated:
https://codereview.chromium.org/396063007

It was reverted because pixelated did not work for accelerated elements.
This is now fixed.

Intent to ship thread:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/zSasd2LL8Mc/discussion

BUG=134040,317991

Review URL: https://codereview.chromium.org/806413006

git-svn-id: svn://svn.chromium.org/blink/trunk@187505 bbb929c8-8fbe-4397-9dbb-9b2b20218538