Add service unittest for WakeLockServiceImpl.

To avoid let service unittest depend on "xvfb", we use the
FakeWakeLockServiceImpl which doesn't create the real
PowerSaveBlocker instance during testing.

BUG=689410

Review-Url: https://codereview.chromium.org/2883903002
Cr-Commit-Position: refs/heads/master@{#477200}
11 files changed