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When a frame is unloading in one process, it must not be visible from the other
processes at some point. Previously, it was done when the frame completed the
unload, now it will be done when it starts unloading.

To achieve that, the RenderFrameProxyHost are removed when the frame starts
unloading. Moreover, no new RenderFrameProxyHosts are created in a frame with an
unloading current document.

+ 2 tests added, originally made by lukasza@chromium.org (Thanks!)

Bug: 960006, 950625
Change-Id: I3f6fc405219a08d9d61f5c0ed8772601f9dc8835
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599182
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659004}