Fix nullptr dereference in ChildControllerImpl
Fix nullptr dereference in ChildControllerImpl

This safeguards against ChildControllerImpl's Unblocker trying
to unblock more than once, which can lead to a nullptr deref.

BUG=None
R=ben@chromium.org

Review URL: https://codereview.chromium.org/1768003002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379495}

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