Revert "When creating emptyp MipMap proxies, don't instantiate them immediately."

This reverts commit 8242c5c199f5d04e4209222b265f9e27f7c55fa7.

Reason for revert: Hitting assert on intel bots on skbug6850overlay2.skp, SkASSERT(proxy->getUniqueKey().isValid()); in processInvalidProxyUniqueKey

Original change's description:
> When creating emptyp MipMap proxies, don't instantiate them immediately.
> 
> This chnages makes it match how we handle non mipped proxies where we
> don't actually instantiate them until we need to.
> 
> Bug: skia:
> Change-Id: Id0c50eefce43ef1458a3ff0bb1881a817b045279
> Reviewed-on: https://skia-review.googlesource.com/106966
> Commit-Queue: Greg Daniel <egdaniel@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>

TBR=egdaniel@google.com,robertphillips@google.com

Change-Id: I1fa6165b69c5bbb1d6bb10abba33dcdb55a27ba3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/107263
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
8 files changed