rebaseline_server: use just skpdiff, not Python Image Library

BUG=skia:2414
R=djsollen@google.com, borenet@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/325413003
diff --git a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
index 1422505..2e5ab8f 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
+++ b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
@@ -36,7 +36,8 @@
     return "different_pixels";
 }
 
-bool SkDifferentPixelsMetric::diff(SkBitmap* baseline, SkBitmap* test, bool computeMask,
+bool SkDifferentPixelsMetric::diff(SkBitmap* baseline, SkBitmap* test, bool computeAlphaMask,
+                                   bool computeRgbDiff, bool computeWhiteDiff,
                                    Result* result) const {
     double startTime = get_seconds();