In Debug, SkMutex(pthread) crashes on re-entrant aquire from same thread.

R=mtklein@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/338973006

src/ports/SkMutex_pthread.h

@@ -19,6 +19,7 @@
 // a corresponding static finalizer).
 struct SkBaseMutex {
     void acquire() {
+        SkASSERT(fOwner != pthread_self());  // SkMutex is not re-entrant
         pthread_mutex_lock(&fMutex);
         SkDEBUGCODE(fOwner = pthread_self();)
     }