Fix thumbnail capture timing.

Thumbnails are now captured at a time that makes sense for tab previews
rather than the old NTP timing (which was largely when switching away
from or leaving a page).

Bug: 928954
Change-Id: I79e2ae1bd066de753a6f6b592639f0db09f35fd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531640
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642920}
6 files changed