Add GrTFlagsMask<> wrapper for C++11 bitfield enums

Add GrTFlagsMask<> wrapper for C++11 bitfield enums

Previously, operator& for enum class bitfields was reserved only to
return a bool. e.g:

  if (flags & Flags::kFlag)

This change adds a new template class GrTFlagsMask<> that gets
instantiated by operator~ and allows us to write expressions like:

  flags &= ~Flags::kFlag
  (flags & ~Flags::kFlag1) | Flags::kFlag2

BUG=skia:

Change-Id: I21e5eb9304135e82cdda459e8a833a3489f6beaf
Reviewed-on: https://skia-review.googlesource.com/8563
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>