Handle floats in BlockContainer::inline_content_sizes

Typically, block-level contents are stacked vertically, so this was just
taking the maximum size among all contents. However, floats can be
stacked horizontally, so we need to sum their sizes.

components/layout_2020/sizing.rs

@@ -33,13 +33,20 @@ impl ContentSizes {
         }
     }
 
-    pub fn max(self, other: Self) -> Self {
+    pub fn max(&self, other: Self) -> Self {
         Self {
             min_content: self.min_content.max(other.min_content),
             max_content: self.max_content.max(other.max_content),
         }
     }
 
+    pub fn add(&self, other: &Self) -> Self {
+        Self {
+            min_content: self.min_content.max(other.min_content),
+            max_content: self.max_content + other.max_content,
+        }
+    }
+
     /// Relevant to outer intrinsic inline sizes, for percentages from padding and margin.
     pub fn adjust_for_pbm_percentages(&mut self, percentages: Percentage) {
         // " Note that this may yield an infinite result, but undefined results