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We have already removed all the transferring user activation code, and
this CL removes experimental flag UserActivationPostMessageTransfer.

Bug: 1067307
Change-Id: Ib444f2ce158a19c1d60f971e461c4a4e59c41a80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2291353Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793022}