Tidy up fOwner in SkMutex_pthread.h.

src/ports/SkMutex_pthread.h

Index: src/ports/SkMutex_pthread.h
diff --git a/src/ports/SkMutex_pthread.h b/src/ports/SkMutex_pthread.h
index 662e549f12e65dd29cc6cd2b632327e8a026afea..f71016b31e88a28b3cc1932a2e4bf1d0f0c93abd 100644
--- a/src/ports/SkMutex_pthread.h
+++ b/src/ports/SkMutex_pthread.h
@@ -13,23 +13,27 @@
 #include <errno.h>
 #include <pthread.h>
 
+// This isn't technically portable, but on Linux and Android pthread_t is some sort of int, and
+// on Darwin it's a pointer.  So assuming pthread_self() never returns 0, it works as a sentinel.
+SkDEBUGCODE(static const pthread_t kNoOwner = 0;)
+
 // A SkBaseMutex is a POD structure that can be directly initialized
 // at declaration time with SK_DECLARE_STATIC/GLOBAL_MUTEX. This avoids the
 // generation of a static initializer in the final machine code (and
 // a corresponding static finalizer).
 struct SkBaseMutex {
     void acquire() {
-        SkASSERT(fOwner != pthread_self());  // SkMutex is not re-entrant
+        SkASSERT(0 == pthread_equal(fOwner, pthread_self()));  // SkMutex is not re-entrant
         pthread_mutex_lock(&fMutex);
         SkDEBUGCODE(fOwner = pthread_self();)
     }
     void release() {
         this->assertHeld();
-        SkDEBUGCODE(fOwner = 0;)
+        SkDEBUGCODE(fOwner = kNoOwner;)
         pthread_mutex_unlock(&fMutex);
     }
     void assertHeld() {
-        SkASSERT(pthread_self() == fOwner);
+        SkASSERT(0 != pthread_equal(fOwner, pthread_self()));
     }
 
     pthread_mutex_t fMutex;
@@ -47,6 +51,7 @@ public:
                 print_pthread_error(status);
                 SkASSERT(0 == status);
             }
+            fOwner = kNoOwner;
         )
     }