Added ability to scale empty rects, and scale rects to zero.

Previous to this CL, the SkRRect backing for gfx::RRectF would
not allow scaling rects to zero, or scaling empty rects by any
factor. With this CL, both operations are supported and tested.

Bug: 497522

Change-Id: I2766d92502954ddbc052a3bb3d29f8f07de201d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525055
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641689}