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This replaces the CHECK with an if statement because the RenderProcessHost
is cleaned up before deletion, which means it can be unregistered from the
list of available RenderProcessHosts before being destructed (hence CHECK
firing).

BUG=403618,404715

Review URL: https://codereview.chromium.org/482333003

Cr-Commit-Position: refs/heads/master@{#290757}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290757 0039d316-1c4b-4281-b951-d872f2087c98