Add ServiceManager::GetInstance() and change ServiceManager::Loader to ServiceLoader

BUG=None
TESTS=Existing
[email protected], darin

Review URL: https://codereview.chromium.org/185553012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254783 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed