[LayoutNG] Use cached HasBoxDecorationBackground

Querying ComputedStyle::HasBoxDecorationBackground is rather costly, use
the cached value from LayoutObject instead for now and add a TODO.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I2fc28723066ece5c0059ddd36c0b4e4d641a154f
Reviewed-on: https://chromium-review.googlesource.com/1152509
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579128}

third_party/blink/renderer/core/paint/ng/ng_inline_box_fragment_painter.cc

@@ -61,18 +61,13 @@ void NGInlineBoxFragmentPainter::PaintBackgroundBorderShadow(
   if (inline_box_fragment_.Style().Visibility() != EVisibility::kVisible)
     return;
 
-  // You can use p::first-line to specify a background. If so, the root line
-  // boxes for a line may actually have to paint a background.
-  bool should_paint_box_decoration_background;
-  if (inline_box_fragment_.Parent()) {
-    should_paint_box_decoration_background =
-        inline_box_fragment_.Style().HasBoxDecorationBackground();
-  } else {
-    // TODO(kojii): Get from fragment once available.
-    bool is_first_line = false;
-    should_paint_box_decoration_background =
-        is_first_line && line_style_ != style_;
-  }
+  // You can use p::first-line to specify a background. If so, the direct child
+  // inline boxes of line boxes may actually have to paint a background.
+  // TODO(layout-dev): Cache HasBoxDecorationBackground on the fragment like
+  // we do for LayoutObject. Querying Style each time is too costly.
+  bool should_paint_box_decoration_background =
+      inline_box_fragment_.GetLayoutObject()->HasBoxDecorationBackground() ||
+      inline_box_fragment_.PhysicalFragment().UsesFirstLineStyle();
 
   if (!should_paint_box_decoration_background)
     return;