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The previous code caused that dismissing the ToolMenu would dismiss the whole Tab
(Toolbar+content view) and left the App in a corrupted state. (The coordinators were
not stopped but their VC was dismissed).

This is a temporary fix. Marq will submit a more robust solution to this problem later on.

BUG=

Review-Url: https://codereview.chromium.org/2667873002
Cr-Commit-Position: refs/heads/master@{#448040}