Issue acquire barrier in SkRefCnt::unique().

When unique() returns true, it must also issue an acquire barrier.
Note that this change may adversly impact SkPath performance,
but editing SkPaths is already a performance issue.

BUG=chromium:258499

No API changes.
TBR=reed@google.com

Review URL: https://codereview.chromium.org/687293002
1 file changed