Change PL::ClippedClippedAbsoluteBoundingBox to pixel-snap

This makes the code when CompositingOptimizations is enabled match
behavior without it enabled.

Bug: 1115577
Change-Id: Iee854f63d135885e1ae5d02a5f76c1dd41eb51aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613347
Commit-Queue: João Victor Almeida de Aguiar <joalmei@microsoft.com>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841782}

third_party/blink/renderer/core/paint/paint_layer.cc

@@ -1124,7 +1124,11 @@ const IntRect PaintLayer::ClippedAbsoluteBoundingBox() const {
     PhysicalRect mapping_rect = LocalBoundingBoxForCompositingOverlapTest();
     GetLayoutObject().MapToVisualRectInAncestorSpace(
         GetLayoutObject().View(), mapping_rect, kUseGeometryMapper);
-    return EnclosingIntRect(mapping_rect);
+    // We use PixelSnappedIntRect here to match the behavior in
+    // CIU:: UpdateAncestorDependentCompositingInputs, even though the code
+    // in UnclippedAbsoluteBoundingBox and its equivalent in
+    // CIU::UpdateAncestorDependentCompositingInputs uses EnclosingIntRect.
+    return PixelSnappedIntRect(mapping_rect);
   } else {
     return GetAncestorDependentCompositingInputs()
         .clipped_absolute_bounding_box;

third_party/blink/renderer/core/paint/paint_layer_test.cc

@@ -2968,7 +2968,13 @@ TEST_P(PaintLayerOverlapTest,
   EXPECT_EQ(fixed->LocalBoundingBoxForCompositingOverlapTest(),
             PhysicalRect(0, 0, 50, 50));
   EXPECT_EQ(fixed->UnclippedAbsoluteBoundingBox(), IntRect(117, 117, 66, 66));
+  // These values differ because CompositingOptimizationsEnabled takes into
+  // account transforms (the one one the .xform element), whereas
+  // CompositingOptimizationsDisabled (incorrectly) does not.
+  if (!RuntimeEnabledFeatures::CompositingOptimizationsEnabled())
     EXPECT_EQ(fixed->ClippedAbsoluteBoundingBox(), IntRect(117, 117, 66, 66));
+  else
+    EXPECT_EQ(fixed->ClippedAbsoluteBoundingBox(), IntRect(118, 118, 64, 64));
 }
 
 TEST_P(PaintLayerOverlapTest, NestedFixedUsesExpandedBoundingBoxForOverlap) {