Allow simulate_mouse_click to work with webrender.
Instead of dispatching to a specific layer, call the dispatch
functions for mouse (move) events that handle whether webrender
is enabled or not.
<!-- 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/13610)
<!-- Reviewable:end -->
Removed the session history from BrowsingContext.
<!-- Please describe your changes on the following line: -->
The session history is stored in the constellation and accessed via the `History` object, so `BrowsingContext` doesn't need to track it.
cc @jdm @ConnorGBrewster @Ms2ger
---
<!-- 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 do not require tests because it's a refactoring.
<!-- 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/13580)
<!-- Reviewable:end -->
Removed duplicate #![feature(plugin)]
<!-- Please describe your changes on the following line: -->
Removed duplicate ` #![feature(plugin)]` from two files.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix#13581 (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it's removing a few lines.
<!-- 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/13585)
<!-- Reviewable:end -->
