Auto merge of #18175 - stshine:no-floats-in-flexbox, r=mbrubeck

layout: Stop call normal block size assignment for flex container

No need to call the `assign_block_size_block_base()` method of normal block in flex.rs since it is implemented for CSS2.

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- [X] These changes fix #14813 (github issue number if applicable).

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components/layout/block.rs

@@ -1429,6 +1429,9 @@ impl BlockFlow {
     /// Determines the type of formatting context this is. See the definition of
     /// `FormattingContextType`.
     pub fn formatting_context_type(&self) -> FormattingContextType {
+        if self.is_inline_flex_item() || self.is_block_flex_item() {
+            return FormattingContextType::Other
+        }
         let style = self.fragment.style();
         if style.get_box().float != float::T::none {
             return FormattingContextType::Other