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In https://crrev.com/2613151 we waited until the view was to send the
message. Which when creating the view and the main frame is correct, but
when it gets navigated it isn't what we want.

A new scenario crept up with opening a preview image first, and then
loading a preview page. This caused the RenderFrameCreated gets called
before the main frame mojo channel is attached and the message is dropped.

To mitigate this use a different callback (DocumentAvailableInMainFrame)
which guarantees the main frame has been attached.

BUG=1158712

Change-Id: I91f61ebec049690bc0fa8ef053612dd985c12443
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622276Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842532}