Add a static Google Doodle to the experimental app list start page.

This CL adds a static Google Doodle to the experimental app list start
page by downloading a JSON blob and using the contained doodle URL as an
image source in the app list start page WebContents.

BUG=453205

Review URL: https://codereview.chromium.org/871193006

Cr-Commit-Position: refs/heads/master@{#314442}
7 files changed