Auto merge of #12292 - ConnorGBrewster:runnable_variant, r=KiChjang

Consolidate runnable variants on DOMManipulationTask into a single Runnable variant.

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Consolidates the runnable variants on `DOMManipulationTask` into a single `Runnable` variant.
Also combines `MainThreadRunnable` into the `Runnable` trait.

I plan on filing a few E-Easy issues after this lands to implement `name` on each of the structs that implement `Runnable`.

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