Auto merge of #28635 - negator:patch-2, r=jdm

Do not use ParallelIterator if not using rayon

The rayon [`ParalllelIIterator`](https://github.com/rayon-rs/rayon/blob/master/src/iter/plumbing/README.md) is being used for inline content size calculations irrespective of the `use_rayon` flag, which can lead to an unbounded (dynamically determined) number of threads being spawned during iteration.

The check is performed for static position calculations, however:
6fced22e47/components/layout_2020/flow/mod.rs (L235-L242)

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components/layout_2020/flow/mod.rs

@@ -141,13 +141,21 @@ impl BlockContainer {
         writing_mode: WritingMode,
     ) -> ContentSizes {
         match &self {
-            Self::BlockLevelBoxes(boxes) => boxes
+            Self::BlockLevelBoxes(boxes) if layout_context.use_rayon => boxes
                 .par_iter()
                 .map(|box_| {
                     box_.borrow_mut()
                         .inline_content_sizes(layout_context, writing_mode)
                 })
                 .reduce(ContentSizes::zero, ContentSizes::max),
+            Self::BlockLevelBoxes(boxes) => boxes
+                .iter()
+                .map(|box_| {
+                    box_.borrow_mut()
+                        .inline_content_sizes(layout_context, writing_mode)
+                })
+                .reduce(ContentSizes::max)
+                .unwrap_or_else(ContentSizes::zero),
             Self::InlineFormattingContext(context) => {
                 context.inline_content_sizes(layout_context, writing_mode)
             },