Rename function in PathOpsExtendedTest which takes strings.

Rename function in PathOpsExtendedTest which takes strings.

A few methods in PathOpsExtendedTest were changed to write to SkStrings
instead of SkStreams. However, the names of these functions confusingly
still have "Stream" in their names. This CL is just a static function
rename and some small clean-up.

Change-Id: Idf21b2aba28a2f984ef30cb5c18e26a43a9c7201
Reviewed-on: https://skia-review.googlesource.com/6819
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>