Placeholder preview image for hover cards (first pass).

Note: This is all behind a flag.

Still to do:
* Switch to og:image or screenshot instead of favicon where possible.
* Shrink bubble margins around the image to UX spec.
* Use more appealing image scaling mode.
* There is one frame of pop when the card switches to a new tab where
  the old image is still visible (this is a hover card rendering issue).

Bug: 928954
Change-Id: I86b7cf9af1fa7d5b64a884a3f39e12ec40b21457
Reviewed-on: https://chromium-review.googlesource.com/c/1459978
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630905}
2 files changed