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This is a quick hack to make the initializeChildFrame message go through
RenderFrameProxy rather than RenderFrameImpl, since the RenderFrameHost
is deleted before it can receive the message.

We can clean up the hack by moving initializeChildFrame to
WebRemoteFrameClient and removing WebFrame::setIsRemote.

BUG=416102,407160
TEST=Simple OOPIF paints in --site-per-process.

Review URL: https://codereview.chromium.org/591503003

Cr-Commit-Position: refs/heads/master@{#295813}