Put in a DCHECK validating that ClipPathClip and ClipPathEffect are mutually exclusive.

This is a small patch that adds DCHECK validation checking that
ClipPathClip and ClipPathEffect are mutually exclusive (as stated in
the description of the fields)

R=pdr@chromium.org, trchen@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I98582c3fab476b1cd743c0398f9e9ebf1aec0da7
Reviewed-on: https://chromium-review.googlesource.com/1225107Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591148}

third_party/blink/renderer/core/paint/object_paint_properties.h

@@ -209,6 +209,8 @@ class CORE_EXPORT ObjectPaintProperties {
     DCHECK(!ScrollTranslation() || !ReplacedContentTransform())
         << "Replaced elements don't scroll so there should never be both a "
            "scroll translation and a replaced content transform.";
+    DCHECK(!ClipPathClip() || !ClipPath())
+        << "ClipPathClip and ClipPathshould be mutually exclusive.";
   }
 #endif