Make listener lists threadsafe with a mutex.

PS1 reverts commit 6c8d242b14355bf66c9137e9e4d6c7861d22168f.
PS2 uses an SkMutex for thread safety.

Change-Id: I9318f92cc028844b3dc5a99a00282c2762057895
Reviewed-on: https://skia-review.googlesource.com/155060
Reviewed-by: Mike Klein <mtklein@google.com>
5 files changed