Avoid null pointer dereference on opening a PagePopup while the renderer process is dying.
Avoid null pointer dereference on opening a PagePopup while the renderer process is dying.

BUG=586999

Review URL: https://codereview.chromium.org/1696123005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#375528}

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