Start using deferred rendering for WebView on L.

Start using deferred rendering for WebView on L.

When running with the synchronous compositor (i.e., WebView), the
deferred rendering strategy requests that all video frames are to
be copied before being sent to the browser compositor.  This is
required for compatibility with the lack of context sharing in
WebView.  It has the benefit over using the copying strategy that
it keeps only ~2 copies at once, rather than 4-5.

Android L supports SurfaceTexture onFrameAvailable callbacks on
user-chosen threads.  This allows us to avoid a deadlock with the
browser compositor.

Prior to L, we continue to use the copying strategy to avoid the
problem instead.

BUG=582170