[Explore Sites] Make composite tile's background transparent

This fixes a bug in dark mode where the background was drawn with
the default Google Grey 100.

Bug: 934489
Change-Id: Icdf9cb6ef6c83f1bafd74f99b5331e518117aa26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625920
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Peter Williamson <petewil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662675}
2 files changed