GitLab

Changed thread_checker usage on VideoCaptureImpl and VideoCaptureImplManager to make it more explicit that the calls are run on the IO thread.

BUG=463009
TEST=Succesfully ran VideoCaptureImplTest.* and VideoCaptureImplManagerTest.* in content_unittests.

Review URL: https://codereview.chromium.org/978993002

Cr-Commit-Position: refs/heads/master@{#322051}