Add defensive code to PaintLayerClipper to avoid crashing when there is no

fragment.

Bug: 750500
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Idc4608f9f871e912458310992703455d9d7ab4cf
Reviewed-on: https://chromium-review.googlesource.com/703460Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506865}

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

@@ -322,6 +322,10 @@ void PaintLayerClipper::CalculateRects(
     const LayoutPoint* offset_from_root) const {
   if (use_geometry_mapper_) {
     DCHECK(fragment_data);
+    // TODO(chrishtr): find the root cause of not having a fragment and fix
+    // it.
+    if (!fragment_data)
+      return;
     CalculateRectsWithGeometryMapper(context, *fragment_data, paint_dirty_rect,
                                      layer_bounds, background_rect,
                                      foreground_rect, offset_from_root);