[Extensions] Reduce the percentage of visible pixels required.

This CL also adds a new utility function to render the icon to be
checked. A follow-up CL will add a unit test to reproduce the analysis
that led to lowering this number.

Bug: 805600
Change-Id: I7b93e1eececa4bb70659d8a89f782d0d9cbc5f44
Reviewed-on: https://chromium-review.googlesource.com/c/1431478
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626215}
2 files changed