Add the ability to enable/disable GPU path renderers

Add the ability to enable/disable GPU path renderers

Adds a bitfield to GrContextOptions that masks out path renderers.
Adds commandline flags support to set this bitfield in tools apps.
Removes GrGLInterfaceRemoveNVPR since we can now accomplish the same
thing in the context options.

BUG=skia:

Change-Id: Icf2a4df36374b3ba2f69ebf0db56e8aedd6cf65f
Reviewed-on: https://skia-review.googlesource.com/8786
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>