Bug 1809568 - Part 2: Provide GetNaturalBaselineBOffset with baseline export context. r=emilio

Some baseline exports are context-sensitive. One example: In line-layout scenario,
the last baseline of a scroll container is always the margin-end. In other (e.g.
flex, grid) scenarios, it's the border-box clamped offset to the last line in the
container.
This enables the required 3 different behaviours for `inline-block` scroll containers
for 3 different `baseline-source` values:
- `auto`: Last baseline, margin-end
- `first`: Border-box clamped offset to the first line
- `last`: Border-box clamped offset to the last line

Differential Revision: https://phabricator.services.mozilla.com/D173886

layout/forms/nsFieldSetFrame.cpp

@@ -851,7 +851,8 @@ nscoord nsFieldSetFrame::SynthesizeFallbackBaseline(
 }
 
 Maybe<nscoord> nsFieldSetFrame::GetNaturalBaselineBOffset(
-    WritingMode aWM, BaselineSharingGroup aBaselineGroup) const {
+    WritingMode aWM, BaselineSharingGroup aBaselineGroup,
+    BaselineExportContext aExportContext) const {
   if (StyleDisplay()->IsContainLayout()) {
     // If we are layout-contained, our child 'inner' should not
     // affect how we calculate our baseline.
@@ -862,7 +863,8 @@ Maybe<nscoord> nsFieldSetFrame::GetNaturalBaselineBOffset(
     return Nothing{};
   }
   MOZ_ASSERT(!inner->GetWritingMode().IsOrthogonalTo(aWM));
-  const auto result = inner->GetNaturalBaselineBOffset(aWM, aBaselineGroup);
+  const auto result =
+      inner->GetNaturalBaselineBOffset(aWM, aBaselineGroup, aExportContext);
   if (!result) {
     return Nothing{};
   }