Use skia::RefPtr to avoid having to manually refcount GraphicsContextState::m_looper.

BUG=251744,254509

Review URL: https://chromiumcodereview.appspot.com/17448009

git-svn-id: svn://svn.chromium.org/blink/trunk@154263 bbb929c8-8fbe-4397-9dbb-9b2b20218538