Listen for DidStopLoading in NavigateIframeToURL.
Listen for DidStopLoading in NavigateIframeToURL.

This wasn't possible before, but we now wait to generate the
DidStopLoading until all frames (including out-of-process iframes)
finish loading.

BUG=436250
TEST=Existing tests still pass.

Review URL: https://codereview.chromium.org/1522443004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365287}

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