Fixed the download intervention in sandbox for RemoteFrame::Navigate path.

Also, fixed test assertions that verify download is not happening: previously we
check the UMA too soon before download decision is made, which may give us false
negatives. The fix is to explicitly wait for
OnDidFinishSubFrameNavigation(), and check the UMA after the waiting.

Bug: 539938
Change-Id: Ib1a829420d68969877454413d0e2c7390a323b21
Reviewed-on: https://chromium-review.googlesource.com/c/1406153
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625557}
12 files changed