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- Adds a feature and corresponding flags in order to gate the integration.
- Modifies ARQuickLookTabHelper to inform delegate only if download was
  successful.
- Hooks up ARQuickLookTabHelper in DownloadServiceManager.
- Sets up ARQuickLookCoordinator and ARQuickLookTabHelper in browser
  coordinator.
- Replaces the test USDZ file with a smaller one.
- Adds EG tests.

Bug: 908696
Change-Id: I37482d356b2e5db5ac09f46df957a3b0cdc48879
Reviewed-on: https://chromium-review.googlesource.com/c/1351980
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612564}