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This is a reland of a7c54a0f

Original change's description:
> Fix a crash in WebUIMessageHandler::AllowJavascript
>
> If WebUI's JS is sending the message when we navigate cross-site,
> then we use the new process ID from WebContents in
> WebUIImpl::CanCallJavascript and a CHECK crash will happen.
>
> This CL changes to use the old process ID from RFH in
> WebUIImpl::CanCallJavascript.
>
> Bug: 1110276
> Change-Id: Ie827060b637dbd3aa4c7ad771c0cb4862763ced2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2326050
> Reviewed-by: Nasko Oskov <nasko@chromium.org>
> Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
> Cr-Commit-Position: refs/heads/master@{#793816}

Bug: 1110276, 1112189, 1112238
Change-Id: I02916364f8cb3871ab4cdf17cd2827a5f1cfbb60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335213Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/master@{#796293}