This replaces the texture creation/caching functions on GrContext with a GrTexturePro...

This replaces the texture creation/caching functions on GrContext with a GrTextureProvider interface. The goal is to pass this narrowly focused object in places that currently take a GrContext but don't need and shouldn't use its other methods. It also has an extended private interface for interacting with non-texture resource types.

Review URL: https://codereview.chromium.org/1107973004