Revert "Tie mip map cache purging to Images, not Bitmaps/PixelRefs"

This reverts commit 11422c63d36bb8a8933e753b1e37ef4615d6be9e.

Reason for revert: Perf regression

Original change's description:
> Tie mip map cache purging to Images, not Bitmaps/PixelRefs
> 
> We were potentially using different keys for lookup vs. add, because we
> were adding with a key based on the pixelRef, which may not have the
> same uniqueID as the originating image.
> 
> Bug: skia:
> Change-Id: Ib4d3d5ead9f5a574cf6d1920080bc9c4ae66c1d0
> Reviewed-on: https://skia-review.googlesource.com/c/161625
> Reviewed-by: Mike Klein <mtklein@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>

TBR=mtklein@google.com,brianosman@google.com,reed@google.com

Change-Id: I9bdce1c6b1d9bf13cc477d2e7f20f7cc0f204b2f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/161828
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed