Remove SkCanvas::flush() and related code from RecordingCanvas

In http://review.skia.org/716476, Skia removed SkCanvas::flush()
and SkCanvas::onFlush().

Flushing is handled externally in SkiaOpenGLPipeline::draw() so
there's no need to record it in the DisplayListData.


Change-Id: I04e71041841d9d9f60acce3fe299cd643d8ae8ea