[Home] Use white doodle background when Chrome Home is enabled

Use a white background, rather than gray, for the Google doodle
when Chrome Home is enabled. Also renames the "transparent"
param to "gray_background" in several methods to more accurately
describe what is being requested.

BUG=701145

Review-Url: https://codereview.chromium.org/2760003002
Cr-Commit-Position: refs/heads/master@{#458244}
7 files changed