reduce code size in GrGLTestInterface.o and GrGLAssembleInterface.o

All these std::function instantiations fill the object files up with
almost 1MB of boilerplate generated code.  I think we can do better by
implementing GrGLFunction as its own std::function-like type with much
less overhead, bringing the total object file size down to about 200K.

This reduces DM from 28780344 to 27726144 on my Mac, a touch above 1MB.

Change-Id: I219a86737d1dfb8b68e4eb47b51a8a98b18ff282
Reviewed-on: https://skia-review.googlesource.com/20551
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed