experimental/editor: mouse drag select, modifierkeys cleanup.

Change-Id: I8c8de54ad6309424bdf18987ccf3eac6bdd41c19
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233080
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
5 files changed