Auto merge of #21181 - nupurbaghel:update_source_set, r=jdm

Implement update source set & select image source

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #11416 (github issue number if applicable).

- [x] These changes require tests but cannot be written until implementation of responsive images is complete

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components/style/values/specified/source_size_list.rs

@@ -18,6 +18,7 @@ use values::specified::{Length, NoCalcLength, ViewportPercentageLength};
 /// A value for a `<source-size>`:
 ///
 /// https://html.spec.whatwg.org/multipage/#source-size
+#[derive(Debug)]
 pub struct SourceSize {
     condition: MediaCondition,
     value: Length,
@@ -38,6 +39,7 @@ impl Parse for SourceSize {
 /// A value for a `<source-size-list>`:
 ///
 /// https://html.spec.whatwg.org/multipage/#source-size-list
+#[derive(Debug)]
 pub struct SourceSizeList {
     source_sizes: Vec<SourceSize>,
     value: Option<Length>,
@@ -52,6 +54,11 @@ impl SourceSizeList {
         }
     }
 
+    /// Set content of `value`, which can be used as fall-back during evaluate.
+    pub fn set_fallback_value(&mut self, width: Option<Length>) {
+        self.value = width;
+    }
+
     /// Evaluate this <source-size-list> to get the final viewport length.
     pub fn evaluate(&self, device: &Device, quirks_mode: QuirksMode) -> Au {
         let matching_source_size = self.source_sizes