webrtc_video_engine_unittest.cc: fix using directives.

There was quite a lot of excessive qualification in the test code.
Cleaned up by adding a few using directives.

BUG=None

Change-Id: I03487f457f15f663acf276ce2a4a9cbbc9804101
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170467
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30806}