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This is the first step toward making remote frames able to be navigated by their parent correctly, and to enable them to transition back into the same process as the parent.

This, combined with https://codereview.chromium.org/573353002/, "just work" to route a navigation request from a RemoteFrame back to its LocalFrame. However, we incorrectly commit the load into the existing LocalFrame (rather than transitioning processes again) and we assert during that commit.

BUG=

Review URL: https://codereview.chromium.org/574403002

Cr-Commit-Position: refs/heads/master@{#297513}