update skpdiff visualization (image magnification with alpha mask)

R=epoger@google.com

Review URL: https://codereview.chromium.org/29103005

git-svn-id: http://skia.googlecode.com/svn/trunk@12174 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
index f2a02c1..b3f5d2d 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
+++ b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
@@ -47,6 +47,10 @@
     return "different_pixels";
 }
 
+bool SkDifferentPixelsMetric::enablePOIAlphaMask() {
+    return false;
+}
+
 int SkDifferentPixelsMetric::queueDiff(SkBitmap* baseline, SkBitmap* test) {
     int diffID = fQueuedDiffs.count();
     double startTime = get_seconds();