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Move initializeChildFrame from RenderFrameImpl to RenderFrameProxy.  Replace RenderFrameImpl::OnSwapOut's call to setIsRemote with WebRemoteFrame::initializeFromFrame that was introduced in part 1 of this change (https://codereview.chromium.org/591413004/)

This will be followed by another CL that will remove setIsRemote on the blink side.

BUG=416102

Review URL: https://codereview.chromium.org/592263002

Cr-Commit-Position: refs/heads/master@{#296822}