disable single threaded debugging code

Stroking has some debug code that is examined
by StrokerTest.cpp; this test is not thread-safe.

Chrome detects this in TSAN, so disable the
test code in SkStroker.cpp the same way it
is disabled in StrokerTest.cpp

R=bsalomon@google.com
Bug: 749315
Change-Id: I6e424648b4d28509789e3e4123112e0fc95e34ed
Reviewed-on: https://skia-review.googlesource.com/28780
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Cary Clark <caryclark@skia.org>
2 files changed