Revert "remove loop in powf"

This reverts commit b23d7758dc8a8bba50122168a3a286ba0b64994c.

Reason for revert: will reland with guards for layout tests

Original change's description:
> remove loop in powf
> 
> Looking at the diff, I don't think we'd land the CL in the other direction.
> 
> Bug: chromium:842374
> 
> Change-Id: I9cbfbcbabbe6a3b36c1369e1ab81ff953cfb3c0b
> Reviewed-on: https://skia-review.googlesource.com/128004
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>
> Auto-Submit: Mike Klein <mtklein@chromium.org>

TBR=mtklein@chromium.org,brianosman@google.com

Change-Id: I4b3bfe48701e08e82a8ed45d28ff347f704903dd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:842374
Reviewed-on: https://skia-review.googlesource.com/128040
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
37 files changed