Full implementation of EventSource, complete with closing and reopening streams.
Fixes#8925.
Source-Repo: https://github.com/servo/servo
Source-Revision: 579ab2d99cd8c07a31c3b68a4659c484d5189ada
This is a large-ish PR that contains the following:
* A new directory is created under `components/script/` called `task_source`, which houses all the stuff for different task sources. Note that the ones that I have now aren't exhaustive - there are more task sources than just the generic ones.
* A `DOMManipulationTaskMsg` which eliminates some usage of `Runnable`s to fire events. Instead, they send event information to the `DOMManipulationTaskSource` and lets the `ScriptTask` handle all the event firing.
* Re-added `fn script_chan`, since I can't think of any other way to give `Trusted` values an appropriate sender.
* Rewrote step 7 of [the end](https://html.spec.whatwg.org/multipage/syntax.html#the-end) to make use of the `DOMManipulationTaskSource`
Partial #7959
Source-Repo: https://github.com/servo/servo
Source-Revision: 740965e39f4d62e5807d21734ed9a7a881eca392
