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 b3f5d2d7e0b00c70fb813e5b73ca4df3b5b9430d..2fe5999b734cfc2438d21db1324fe93482718ab4 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
+++ b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
@@ -52,9 +52,14 @@ bool SkDifferentPixelsMetric::enablePOIAlphaMask() {
 }
 
 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.

[scroggo, 2013/11/07 21:43:23]

creation*

+    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;