[LayoutNG] Fix PaintInvalidator to use NGPaint code path for NGMixin

This patch fixes PaintInvalidator to use NGPaint code path not only
for LayoutNGBlockFlow but also for LayoutNGMixin.

This fixes inline children in table cells not be painted at all.
~700 tests turn to green.

Bug: 714962
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I36fec7263b2a1e1d06271c9f672f9e0e12b25230
Reviewed-on: https://chromium-review.googlesource.com/776177
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517497}

third_party/WebKit/Source/core/paint/PaintInvalidator.cpp

@@ -567,8 +567,7 @@ void PaintInvalidator::InvalidatePaint(
     }
   }
 
-  if (RuntimeEnabledFeatures::LayoutNGEnabled() &&
-      object.IsLayoutNGBlockFlow()) {
+  if (RuntimeEnabledFeatures::LayoutNGEnabled() && object.IsLayoutNGMixin()) {
     // If the LayoutObject has a paint fragment, it means this LayoutObject and
     // its descendants are painted by NG painter.
     // In the inline NG paint phase, this is a block flow with inline children.