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As the first step in making coordinators more consistent in their
public APIs and internal usage of Browser, CommandDispatcher is moving
to be created and owned by Browser instances.

Note that this still leaves us with two dispatchers per scene; one for
regular tabs and one for OTR tabs. Untangling that is a separate (and
more painful) effort.

This change should have no direct side effects

Change-Id: I81211a33616cf3c47cc8c286da09166bf1003d8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1940244
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720017}