SkVarAlloc::approxBytesAllocated()

This is what I was getting at on the other CL.

BUG=skia:

Review URL: https://codereview.chromium.org/730193003
rebaseline perspective gm after matrix dont-renormalize-cl

BUG=skia:
TBR=
NOTRY=True

Review URL: https://codereview.chromium.org/741433002
minor cleanup in GrGpu.h

Review URL: https://codereview.chromium.org/731073003
Revert of SkVarAlloc::approxBytesAllocated (patchset #5 id:80001 of https://codereview.chromium.org/730193003/)

Reason for revert:
Android needs dlmalloc_usable_size().

Original issue's description:
> SkVarAlloc::approxBytesAllocated()
>
> This is what I was getting at on the other CL.
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/f27f1bcce50c8f95aea8469684a70b70c9baee09
>
> CQ_EXTRA_TRYBOTS=Test-Android-Nexus5-Adreno330-Arm7-Release-Trybot

TBR=reed@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/741443002