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When the WebView app makes a call to java over the JavaScriptBridge,
we leave the renderer hanging on a synchronous IPC. Once control
is passed into Java, it's possible that the WebView may get destroyed
(and thus the IPC channel back to the renderer closed) which means we
can't unblock the renderer waiting on the IPC response.

Instead we cache the IPC reply message and while waiting on Java to come
back to us, if we detect that the RenderFrame has been deleted, send a
reponse back before the IPC channel is closed.

BUG=408188

Review URL: https://codereview.chromium.org/498633003

Cr-Commit-Position: refs/heads/master@{#292631}