fix multithread related crashes in skpdiff

tools/skpdiff/SkPMetric.cpp

Index: tools/skpdiff/SkPMetric.cpp
diff --git a/tools/skpdiff/SkPMetric.cpp b/tools/skpdiff/SkPMetric.cpp
index a19ed826f8f1cd344708a65dd51ade5beb4b6c0d..5b770b08bdbdee7e5803eb36a9dd317d4d69b6c9 100644
--- a/tools/skpdiff/SkPMetric.cpp
+++ b/tools/skpdiff/SkPMetric.cpp
@@ -430,8 +430,14 @@ const char* SkPMetric::getName() {
 
 int SkPMetric::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_back();
+    queueLock.release();
+
     diff.result = 0.0;
 
     // Ensure the images are comparable
@@ -456,7 +462,8 @@ int SkPMetric::queueDiff(SkBitmap* baseline, SkBitmap* test) {
 
 
 void SkPMetric::deleteDiff(int id) {
-
+    // clean up the POI array to save space
+    return fQueuedDiffs[id].poi.reset();
 }
 
 bool SkPMetric::isFinished(int id) {