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This is a temporary fix so that the intervention does not break back
button for pdfs. The root cause is that mouse click on pdf is not considered
a user gesture (crbug.com/934637). Once this is fixed, then this CL's changes
can be removed.

Bug: 965434
Change-Id: I15d8392e98d1578a1c30f242c847fbcf8e4bd8ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623137
Commit-Queue: Shivani Sharma <shivanisha@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662831}