Rename GrDrawingMgr::abandon to wasAbandoned & add a matching entry point to GrDrawingContext

Since GrDrawingContext is generally replacing GrRenderTarget, this is to provide functionality similar to GrRenderTarget::wasDestroyed.

This is split out of https://codereview.chromium.org/1930013002/ ((Mostly) Retract GrRenderTarget from SkGpuDevice)

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Review-Url: https://codereview.chromium.org/1979913002
7 files changed