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This fully enables support for context menu in iFrames.

Bug: 873660, 873662
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I3345f56f6812ef619c6ed5b7bcabd34e09de410f
Reviewed-on: https://chromium-review.googlesource.com/1174052
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583053}