[NTP] Calc and store custom background color V3.

This CL relands https://chromium-review.googlesource.com/c/chromium/src/+/1612235
with following changes:
- calculates image color async, as it can take 1-2 seconds,
- fixes the image attribution bug.

The seconds patch corresponds to the reverted CL.

Bug: 874325
Change-Id: Iec9812c93dc383043b22048ec4387f1f08a28e5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618502
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661606}
9 files changed