Bug fix for cull_line in SkDashPathEffect.

In cull_line we must also check if both points are the same. Otherwise we
fail the assert in the else "SkASSERT(dy && !dx)".

This is currently blocking the roll as it fails a webkit test.

BUG=skia:

Review URL: https://codereview.chromium.org/703783002
diff --git a/src/effects/SkDashPathEffect.cpp b/src/effects/SkDashPathEffect.cpp
index a053066..412965e 100644
--- a/src/effects/SkDashPathEffect.cpp
+++ b/src/effects/SkDashPathEffect.cpp
@@ -66,7 +66,7 @@
     SkScalar dx = pts[1].x() - pts[0].x();
     SkScalar dy = pts[1].y() - pts[0].y();
 
-    if (dx && dy) {
+    if ((dx && dy) || (!dx && !dy)) {
         return false;
     }