Optimize visualbench offscreen blits

Modifies NvprWrappedBenchmark to present its offscreen canvas via
GrContext::copySurface rather than going through the SkCanvas API.
Only copies a subrectangle the size of the benchmark, not the entire
canvas.

BUG=skia:

Review URL: https://codereview.chromium.org/1443263002
4 files changed