GitLab

Move the setup from BrowserActionsContainerTest into a
platform-independent test, so that we also start to get
test coverage on Mac.  In this first patch, move the
infrastructure and the basic test.  More to follow.

BUG=422381

Review URL: https://codereview.chromium.org/640423003

Cr-Commit-Position: refs/heads/master@{#299313}