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Removing this call should be safe because the call just does following
things:
1. Sets started_ to false in VideoCapturerTrackSource. This doesn't
matter since it will be destroyed soon.
2. Calls WebRtcDummyVideoCapturer::Stop. This just sets the state of the
dummy video captuer to CS_STOPPED. This doesn't matter since it will be
destroyed soon.

Motivation behind this change is that we want to remove Restart and Stop
from VideoTrackSourceInterface. This code is the last usage of these
methods.

Review-Url: https://codereview.chromium.org/2308543002
Cr-Commit-Position: refs/heads/master@{#416290}