Implement shared locks in the style of pthread's rwlock.

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.release();
+    sm.acquireShared();
+    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();
+                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();
+                value += 1;
+                for (int i = 0; i < kSharedSize; ++i) {
+                    shared[i] = value;
+                }
+                sm.release();
+            }
+        }
+    });
+}