[Blink] Add fixed-pos counterscroll in PaintLayerClipper for RLS mode

The caller of PaintLayerClipper expects the returned clip rect to be in
the scrolled space of LayoutView, while the computed clip rect for a
fixed-pos layer is in the unscrolled space prior to compensation. The
fixed-pos compensation has been done for non-RLS mode, this CL adds the
RLS counterpart.

BUG=821279

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I48a92d0614d97c745fe66b1265dce0613514ffc4
Reviewed-on: https://chromium-review.googlesource.com/961643Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Tien-Ren Chen <trchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543584}

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

@@ -635,7 +635,7 @@ void PaintLayerClipper::CalculateBackgroundClipRect(
   if (parent_clip_rects->Fixed() &&
       &context.root_layer->GetLayoutObject() == layout_view &&
       output != LayoutRect(LayoutRect::InfiniteIntRect()))
-    output.Move(LayoutSize(layout_view->GetFrameView()->GetScrollOffset()));
+    output.Move(LayoutSize(layout_view->OffsetForFixedPosition()));
 }
 
 void PaintLayerClipper::GetOrCalculateClipRects(const ClipRectsContext& context,

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

@@ -864,4 +864,56 @@ TEST_F(PaintLayerClipperTest, ScrollbarClipBehaviorParent) {
   EXPECT_EQ(LayoutRect(0, 0, 200, 300), layer_bounds);
 }
 
+class PaintLayerClipperTestParameterized
+    : public ::testing::WithParamInterface<bool>,
+      private ScopedRootLayerScrollingForTest,
+      public PaintLayerClipperTest {
+ public:
+  PaintLayerClipperTestParameterized()
+      : ScopedRootLayerScrollingForTest(GetParam()) {}
+};
+
+INSTANTIATE_TEST_CASE_P(All,
+                        PaintLayerClipperTestParameterized,
+                        ::testing::Bool());
+
+TEST_P(PaintLayerClipperTestParameterized, FixedLayerClipRectInDocumentSpace) {
+  SetBodyInnerHTML(R"HTML(
+    <div style="position:fixed; left:100px; top:200px; width:300px; height:400px; overflow:hidden;">
+      <div id="target" style="position:relative;"></div>
+    </div>
+    <div style="height:3000px;"></div>
+  )HTML");
+
+  Element* target = GetDocument().getElementById("target");
+  PaintLayer* target_layer =
+      ToLayoutBoxModelObject(target->GetLayoutObject())->Layer();
+
+  {
+    ClipRect clip_rect;
+    target_layer->Clipper(PaintLayer::kDoNotUseGeometryMapper)
+        .CalculateBackgroundClipRect(
+            ClipRectsContext(GetDocument().GetLayoutView()->Layer(),
+                             kAbsoluteClipRectsIgnoringViewportClip,
+                             kIgnorePlatformOverlayScrollbarSize,
+                             kIgnoreOverflowClipAndScroll),
+            clip_rect);
+    EXPECT_EQ(LayoutRect(100, 200, 300, 400), clip_rect.Rect());
+  }
+
+  GetDocument().domWindow()->scrollTo(0, 50);
+
+  {
+    ClipRect clip_rect;
+    target_layer->Clipper(PaintLayer::kDoNotUseGeometryMapper)
+        .CalculateBackgroundClipRect(
+            ClipRectsContext(GetDocument().GetLayoutView()->Layer(),
+                             kAbsoluteClipRectsIgnoringViewportClip,
+                             kIgnorePlatformOverlayScrollbarSize,
+                             kIgnoreOverflowClipAndScroll),
+            clip_rect);
+    EXPECT_EQ(LayoutRect(100, 250, 300, 400), clip_rect.Rect());
+  }
+}
+
 }  // namespace blink