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You can't assume that there's a root layer always during layout, even if
there's a composite and readback.  WebTestProxyBase::CapturePixelsAsync
iterates through popups (which may get closed between that call and
Layout).  If it's closed, then its root layer goes away.

BUG=470817

Review URL: https://codereview.chromium.org/1038133003

Cr-Commit-Position: refs/heads/master@{#322611}