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

implemented string-valued text-overflow

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---

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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/style/properties/longhand/text.mako.rs

@@ -12,9 +12,6 @@
                                              Method("has_overline", "bool"),
                                              Method("has_line_through", "bool")]) %>
 
-% if product == "servo":
-    ${helpers.single_keyword("text-overflow", "clip ellipsis", animatable=False)}
-% else:
 <%helpers:longhand name="text-overflow" animatable="False">
     use std::fmt;
     use style_traits::ToCss;
@@ -93,7 +90,6 @@
         }
     }
 </%helpers:longhand>
-% endif
 
 ${helpers.single_keyword("unicode-bidi",
                          "normal embed isolate bidi-override isolate-override plaintext",