Add sample mouse support to Viewer

Bug: skia:6830
Change-Id: I1a6bb781465a29cec4946462e234d63c48693454
Reviewed-on: https://skia-review.googlesource.com/66543
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
4 files changed