Add resource provider flag to avoid client-side buffers

Add resource provider flag to avoid client-side buffers

On some platforms, a newly-created buffer was liable to be CPU backed.
This would break code that expected a VBO (aka instanced rendering).
This change adds an optional flag to GrResourceProvider that requires
a buffer to be created in GPU memory.

It also moves the CPU backing logic into Gr land in order to properly
cache real VBOs on platforms that prefer client-side buffers.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2143333002

Review-Url: https://codereview.chromium.org/2143333002