Battery Status API: fix flaky layout tests.

Some refactoring of the battery status layout tests to get
rid of flakyness. TestRunner::didChangeBatteryStatus is
synchronous now (see https://codereview.chromium.org/342343004/),
and the tests have been modified accordingly. The tests have
also been simplified somewhat by removing cleanupAndFinish()
functions with the setPageVisibility("hidden") calls. The latter
appeared unnecessary, as the battery managers are stopped
automatically between the tests. Before each test gc() is
performed to  make sure no unused battery managers are present.

Also remove battery-status tests from TestExpectations.

BUG=384396

Review URL: https://codereview.chromium.org/348343003

git-svn-id: svn://svn.chromium.org/blink/trunk@176838 bbb929c8-8fbe-4397-9dbb-9b2b20218538