Move memoization of multisample specs id to GrRenderTarget

GrGpu used to perform the memoization on behalf of GrRenderTarget via
a public accessor. This change updates GrRenderTarget to do its own
work.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2467593002

Review-Url: https://codereview.chromium.org/2467593002
9 files changed