[Download Home] Custom tab backgrounds

* Background and toolbar of the Custom Tab should be
  off-black for images and black for all other media.

* Add an extra to the Intent that allows changing the
  background.

* Overrides ChromeActivity methods so that the colors
  can be set at the right times.

* Add a method to disable the toolbar shadow.

BUG=650455

Review-Url: https://codereview.chromium.org/2484053002
Cr-Commit-Position: refs/heads/master@{#430780}