Auto merge of #13924 - cynicaldevil:text-overflow, r=emilio

implemented string-valued text-overflow

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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 #13709

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- [x] There are tests for these changes

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cc @Manishearth
The lorem-ipsum example from MDN works as expected.

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components/layout/fragment.rs

@@ -1037,12 +1037,15 @@ impl Fragment {
     }
 
     /// Transforms this fragment into an ellipsis fragment, preserving all the other data.
-    pub fn transform_into_ellipsis(&self, layout_context: &LayoutContext) -> Fragment {
+    pub fn transform_into_ellipsis(&self,
+                                   layout_context: &LayoutContext,
+                                   text_overflow_string: String)
+                                   -> Fragment {
         let mut unscanned_ellipsis_fragments = LinkedList::new();
         unscanned_ellipsis_fragments.push_back(self.transform(
                 self.border_box.size,
                 SpecificFragmentInfo::UnscannedText(
-                    box UnscannedTextFragmentInfo::new("…".to_owned(), None))));
+                    box UnscannedTextFragmentInfo::new(text_overflow_string, None))));
         let ellipsis_fragments = TextRunScanner::new().scan_for_runs(&mut layout_context.font_context(),
                                                                      unscanned_ellipsis_fragments);
         debug_assert!(ellipsis_fragments.len() == 1);