This is going to be done in multiple steps,
because lots of tests depend on global TestingBrowserProcess.

However, the current solution is very error-prone,
and may lead to state being carried from one test
to another (via NotificationService), resulting in crashes.

In theory the ScopedTestingBrowserProcess could be made
empty for now, but re-initializing g_browser_process inside it
has two benefits:

1) It verifies that changing g_browser_process actually works.

2) It protects at least a subset of tests from carrying state.

BUG=61062
TEST=unit_tests, possibly more

Review URL: http://codereview.chromium.org/6478005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@74926 0039d316-1c4b-4281-b951-d872f2087c98