Use a simpler method for calculating atan

Change-Id: I675851350c02c85f3f212c214766b3e8a6761dc9
Reviewed-on: https://skia-review.googlesource.com/13402
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed