ShapeDetection: Make sure tests are running serially

There could be more than one MockFaceDetection association with the same
MockFaceDetectionProvider, the first mock service is random Gcd in same
context, so ensure these tests run serially.

BUG=707689

Review-Url: https://codereview.chromium.org/2818563006
Cr-Commit-Position: refs/heads/master@{#475335}
2 files changed