fix multithread related crashes in skpdiff

tools/skpdiff/SkDifferentPixelsMetric_cpu.cpp

Index: tools/skpdiff/SkDifferentPixelsMetric_cpu.cpp
diff --git a/tools/skpdiff/SkDifferentPixelsMetric_cpu.cpp b/tools/skpdiff/SkDifferentPixelsMetric_cpu.cpp
index 4e5e93969c3f743ecc6024bbba9576a06a3791ef..ddc36eae21ab4ed078e8bfb6e847f6c389afb5bf 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric_cpu.cpp
+++ b/tools/skpdiff/SkDifferentPixelsMetric_cpu.cpp
@@ -24,8 +24,14 @@ const char* SkDifferentPixelsMetric::getName() {
 
 int SkDifferentPixelsMetric::queueDiff(SkBitmap* baseline, SkBitmap* test) {
     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();
+
     SkTDArray<SkIPoint>* poi = diff->poi = new SkTDArray<SkIPoint>();
 
     // If we never end up running the kernel, include some safe defaults in the result.