[LayoutNG] Fix scrollWidth/scrollLeft on RTL when box is narrow

When the scrollbar is on left and the box is narrower than
the scrollbar, the scrollbar overflows to left.
LayoutBox::AddLayoutOverflow() expects the overflow adjusted
by the amount the scrollbar overflows.

This affects a jQuery plugin that detects RTL scroll type[1].

[1] https://github.com/othree/jquery.rtl-scroll-type

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I70e7b1ecc20857ff10687afd7d5e3b7d70afb2dc
Reviewed-on: https://chromium-review.googlesource.com/1215365Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590138}

third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-features=LayoutNG

@@ -389,7 +389,6 @@ crbug.com/591099 fast/overflow/recompute-overflow-of-layout-root-container.html
 crbug.com/591099 fast/reflections/opacity-reflection-transform.html [ Failure ]
 crbug.com/591099 fast/replaced/table-replaced-element.html [ Failure ]
 crbug.com/591099 fast/scrolling/content-box-smaller-than-scrollbar.html [ Failure ]
-crbug.com/591099 fast/scrolling/jquery-rtl-scroll-type.html [ Failure ]
 crbug.com/591099 fast/scrolling/scrollbar-tickmarks-hittest.html [ Failure Pass ]
 crbug.com/591099 fast/sub-pixel/sub-pixel-border-2.html [ Failure ]
 crbug.com/591099 fast/table/border-collapsing/004-vertical.html [ Failure ]

third_party/blink/renderer/core/layout/ng/layout_ng_mixin.cc

@@ -139,6 +139,17 @@ void LayoutNGMixin<Base>::AddScrollingOverflowFromChildren() {
     }
   }
 
+  // If the scrollbar is on left, and the box is narrower than the scrollbar,
+  // the overflow is shifted to left by the amount the scrollbar overflows.
+  if (Base::ShouldPlaceVerticalScrollbarOnLeft()) {
+    LayoutUnit vertical_scrollbar_width =
+        LayoutUnit(Base::VerticalScrollbarWidth());
+    if (physical_fragment->Size().width < vertical_scrollbar_width) {
+      children_overflow.offset.left +=
+          physical_fragment->Size().width - vertical_scrollbar_width;
+    }
+  }
+
   // LayoutOverflow takes flipped blocks coordinates, adjust as needed.
   LayoutRect children_flipped_overflow = children_overflow.ToLayoutFlippedRect(
       physical_fragment->Style(), physical_fragment->Size());