Add ExtensionsTest base class for extensions_unittests tests

It provides shared setup of the content module, extensions module, and
BrowserContextKeyedServices.

I converted ProcessManagerTest to it as an example.

BUG=395820
TEST=extensions_unittests

Review URL: https://codereview.chromium.org/426233006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@286703 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed