Always make SkImageShaders in heap.

I made a couple of measurments, and it looks like any differences is
well below the noise threshold.

Just for the record run1: .9991 of baseline and run2 .9988 of baseline.
I was using top25 .skps as workload.

TBR=mtklein@google.com

Change-Id: If4fa06e5d5df72fb67dbb4bbb99c926f05765897
Reviewed-on: https://skia-review.googlesource.com/8341
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
8 files changed