Do not calculate many sierpinski fractals for each nanobench run unless needed

Removes work done by the constructors of picture_nesting benches,
and moves the work to the Benchmark::onPreDraw override.

This avoids PictureNesting::sierpinsky showing up in profile traces
when profiling other benches.

Review URL: https://codereview.chromium.org/725523002
Remove unused NVPR related GL tokens

Remove unused NVPR related GL tokens.

Also replace GR_GL_PATH_INITIAL_END_CAP and GR_GL_PATH_TERMINAL_END_CAP
with a single call setting GR_GL_PATH_END_CAPS. Skia does not and
probably will not have different initial and terminal caps. This came up
in the review of command buffer implementation of the extension.

If more NVPR features will be used, the respective tokens can be added
back per implemented feature.

Review URL: https://codereview.chromium.org/723453002