GitLab

* Force the Custom Tab Intent to use the same package as Chrome
  when opening the media viewer.

* Open media downloads inside of the internal media viewer when
  opening the file in the notification or via the snackbar.

* Shuffle code around for creating media viewer Intents in the
  hopes of consolidating them in places that makes sense.

BUG=650448

Review-Url: https://codereview.chromium.org/2376893006
Cr-Commit-Position: refs/heads/master@{#422624}