Add CHECK for null WebState in CRWWebController.
Add CHECK for null WebState in CRWWebController.

This check was added in an effort to better understand the conditions
in which the crash reported in the referenced bug is occurring.  Since
|_webStateImpl| is only ever reset in |-close|, it is likely that the
WebState has been corrupted somehow.

BUG=593852

Review URL: https://codereview.chromium.org/1879933002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386720}

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