Add clear function to TestAtlasTextRendering interface

Bug: skia:
Change-Id: I24563abe5e72e57e6764ddf6aca76ccf098cc488
Reviewed-on: https://skia-review.googlesource.com/73640
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed