Auto merge of #15129 - cynicaldevil:iframe-resize, r=emilio

Refactor to send iframe resize messages directly from layout thread to constellation

<!-- Please describe your changes on the following line: -->

---
<!-- 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 #14682.
<!-- Either: -->

r? @jdm
passing tests:
tests/wpt/mozilla/tests/css/matchMedia.html, tests/wpt/mozilla/tests/mozilla/window_resize_not_triggered_on_load.html, tests/wpt/mozilla/tests/mozilla/iframe/resize_after_load.html, tests/wpt/mozilla/tests/css/meta_viewport_resize.html

<!-- 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/15129)
<!-- Reviewable:end -->

components/script_traits/lib.rs

@@ -696,8 +696,6 @@ pub enum WebDriverCommandMsg {
 pub enum ConstellationMsg {
     /// Exit the constellation.
     Exit,
-    /// Inform the constellation of the size of the viewport.
-    FrameSize(PipelineId, Size2D<f32>),
     /// Request that the constellation send the FrameId corresponding to the document
     /// with the provided pipeline id
     GetFrame(PipelineId, IpcSender<Option<FrameId>>),