fix multithread related crashes in skpdiff

tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp

Index: tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
diff --git a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
index f2a02c1c1823972793a9af7685910e6d46ec4d06..dcb99dd9d6494ffea4d484ad6f45e2c178861bb1 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
+++ b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
@@ -48,9 +48,14 @@ const char* SkDifferentPixelsMetric::getName() {
 }
 
 int SkDifferentPixelsMetric::queueDiff(SkBitmap* baseline, SkBitmap* test) {
-    int diffID = fQueuedDiffs.count();
     double startTime = get_seconds();
+
+    // since we currently synchronously do the diff within queueDiff we only need
+    // to protect the createion of the ID and queuedDiff object.
+    SkAutoMutexAcquire queueLock(fQueueMutex);
+    int diffID = fQueuedDiffs.count();
     QueuedDiff* diff = fQueuedDiffs.push();
+    queueLock.release();
 
     // If we never end up running the kernel, include some safe defaults in the result.
     diff->finished = false;