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Run with DriveFS enabled:
- Drive-related context_menu.js tests.
- Drive-related metadata.js tests.
- folder_shortcuts.js tests.
- fileDisplayDrive in tablet mode with DriveFS enabled.

In order to make those tests pass:
- Pass capabilities metadata to FakeDriveFs in SetMetadata() and return
  it in GetMetadata().
- Populate croppedThumbnailUrl so metadata cache doesn't retry fetching
  metadata, trying to get a value for it.
- Remove two unused and incomplete tests in context_menu.js.
- Only perform early-init for the downloads volume for the "user"
  profile.

Bug: 891197
Change-Id: I8ddcbd75b74258b6988ad5c70c4e70c707d18f70
Reviewed-on: https://chromium-review.googlesource.com/c/1255086Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596048}