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When the render widget is hidden, the hang monitor timeout is explicitly
stopped. However, tasks executed *after* the widget hide signal may
trigger a restart of the hang monitor timeout, e.g., queued input
events. Prevent such restarts when the widget is hidden while
also ensuring the hang monitor restarts when the widget is shown again.

BUG=458594

Review URL: https://codereview.chromium.org/938123003

Cr-Commit-Position: refs/heads/master@{#319778}