[WebRTC Browser Tests] Slight refactoring of the webrtc audio tests.

Lately, fixing some flakyness in the WebRTC Browser tests (crrev.com/c/1445936,
crrev.com/c/1447693) I noticed some inconsistency in the way the tests
are defined. This CL consolidates where the constraints that are used
throughout the tests are defined and should reduce the maintenance burden.

BUG=926830

Change-Id: I63f76e6e720f430f7b25a8326fe4dbb225a3a85a
Reviewed-on: https://chromium-review.googlesource.com/c/1451830
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Armando Miraglia <armax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629132}
2 files changed