Add test for ServiceManager

I also separated out the dynamic service loader from the ServiceManager file, to remove the dependency of the ServiceManager on the shell's context. This made testing cleaner.

BUG=None
R=viettrungluu@chromium.org, viettrungluu

Review URL: https://codereview.chromium.org/122173004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242952 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242952