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When renderer navigates, all worker agent hosts will be eventually
disposed. In order to keep protocol determenistic, this patch proactively
detaches all worker sessions on renderer navigation.

R=dgozman

Change-Id: Ief0fe3db90eade9e12baa3cbe4f6b78785c60e4f
Reviewed-on: https://chromium-review.googlesource.com/c/1345529
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609886}