[Icons NTP] Make Local NTP render largeIconUrl instead of thumbnail, if available.

This is a minimalistic way to display large icons on local NTP for testing.
The code looks for the "largeIconUrl" data, which only becomes available
beyind flag. If not found then use thumbnail as before; otherwise large icons.
New NTP styles are not implemented in this CL.

BUG=467712

Review URL: https://codereview.chromium.org/1013173003

Cr-Commit-Position: refs/heads/master@{#321247}
2 files changed