Eliminate SVGTextRunRenderingContext::m_activePaintingResource

With geometry being emitted independently from the paint server setup, it
becomes possible to just rely on the current state of the GC when
painting SVG font text (which in turn allows us to make stricter
assumptions with more confidence elsewhere.)

Review URL: https://codereview.chromium.org/565063003

git-svn-id: svn://svn.chromium.org/blink/trunk@182153 bbb929c8-8fbe-4397-9dbb-9b2b20218538