media: Update comments in video_capture_device_factory.h

We create the VideoCaptureDeviceFactory instance on the UI thread
now. Update the comments accordingly.

BUG=584797
TEST=None
R=chfremer

Review-Url: https://codereview.chromium.org/2849023002
Cr-Commit-Position: refs/heads/master@{#473422}
1 file changed