Adds getter for ContextFactory to content

Chrome (and others) need this so they can supply the ContextFactory to
the Compositor and get rid of Compositor::GetInstance.

BUG=none
TEST=none
R=piman@chromium.org

Review URL: https://codereview.chromium.org/276973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269447 0039d316-1c4b-4281-b951-d872f2087c98

• content/browser/compositor/gpu_process_transport_factory.cc[diff]
• content/browser/compositor/gpu_process_transport_factory.h[diff]
• content/browser/compositor/image_transport_factory.cc[diff]
• content/browser/compositor/image_transport_factory.h[diff]
• content/browser/compositor/no_transport_image_transport_factory.cc[diff]
• content/browser/compositor/no_transport_image_transport_factory.h[diff]
• content/browser/context_factory.cc[Added - diff]
• content/content_browser.gypi[diff]
• content/public/browser/context_factory.h[Added - diff]