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This change allows the preview tab to tell PermissionRequestManager to
drop any permission requests it receives for the associated WebContents.

TBR=andypaicu@chromium.org

Bug: 1057951
Change-Id: I65ee35fbaf2081cc99d8384a5b0818fc9f026ef3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2112812
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752573}