[AndroidWebViewShell] Add MediaStream API layout tests.
Add two layout tests for the MediaStream API (aka getUserMedia).
The tests check correctness of callbacks for the following cases:
- when permissions are denied
- success case (i.e. getUserMedia succeeds).
This patch also adds the 'shouldBe' and related JavaScript
testing primitives and the ability to grant/deny permissions in
tests.
BUG=515926
Review URL: https://codereview.chromium.org/
1314033003
Cr-Commit-Position: refs/heads/master@{#348142}