Auto merge of #23564 - mmiecz:clipboard-refactoring, r=jdm

Clipboard refactoring

<!-- Please describe your changes on the following line: -->
This PR removes clipboard handling from the constellation. Instead, now embedder handles it.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #23440 (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it is enough to test manually in input box, if copying and pasting still works .

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23564)
<!-- Reviewable:end -->

tests/unit/script/textinput.rs

@@ -8,13 +8,34 @@
 // except according to those terms.
 
 use keyboard_types::{Key, Modifiers};
-use script::clipboard_provider::DummyClipboardContext;
+use script::clipboard_provider::ClipboardProvider;
 use script::test::DOMString;
 use script::textinput::{
     Direction, Lines, Selection, SelectionDirection, TextInput, TextPoint, UTF16CodeUnits,
     UTF8Bytes,
 };
 
+pub struct DummyClipboardContext {
+    content: String,
+}
+
+impl DummyClipboardContext {
+    pub fn new(s: &str) -> DummyClipboardContext {
+        DummyClipboardContext {
+            content: s.to_owned(),
+        }
+    }
+}
+
+impl ClipboardProvider for DummyClipboardContext {
+    fn clipboard_contents(&mut self) -> String {
+        self.content.clone()
+    }
+    fn set_clipboard_contents(&mut self, s: String) {
+        self.content = s;
+    }
+}
+
 fn text_input(lines: Lines, s: &str) -> TextInput<DummyClipboardContext> {
     TextInput::new(
         lines,