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This fixes an issue where if the embedder WebContents is destroyed during a GuestView creation we may end up leaking the guest WebContents.

BUG=419020

Committed: https://crrev.com/52fce9455ffd688ef3752a816192f9e76555e7d1
Cr-Commit-Position: refs/heads/master@{#298934}

Review URL: https://codereview.chromium.org/642573002

Cr-Commit-Position: refs/heads/master@{#299117}