Add asserts for shared mutex.

tests/SkSharedMutexTest.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSharedMutex.h"
 #include "SkTaskGroup.h"
 
 #include "Test.h"
 
 DEF_TEST(SkSharedMutexBasic, r) {
     SkSharedMutex sm;
     sm.acquire();
+    sm.assertHeld();
     sm.release();
     sm.acquireShared();
+    sm.assertHeldShared();
     sm.releaseShared();
 }
 
 DEF_TEST(SkSharedMutexMultiThreaded, r) {
     SkSharedMutex sm;
     static const int kSharedSize = 10;
     int shared[kSharedSize];
     int value = 0;
     for (int i = 0; i < kSharedSize; ++i) {
         shared[i] = 0;
     }
     sk_parallel_for(8, [&](int threadIndex) {
         if (threadIndex % 4 != 0) {
             for (int c = 0; c < 100000; ++c) {
                 sm.acquireShared();
+                sm.assertHeldShared();
                 int v = shared[0];
                 for (int i = 1; i < kSharedSize; ++i) {
                     REPORTER_ASSERT(r, v == shared[i]);
                 }
                 sm.releaseShared();
             }
         } else {
             for (int c = 0; c < 100000; ++c) {
                 sm.acquire();
+                sm.assertHeld();
                 value += 1;
                 for (int i = 0; i < kSharedSize; ++i) {
                     shared[i] = value;
                 }
                 sm.release();
             }
         }
     });
 }