Support multiprocess constellation on Windows

[vvuk]

Allow -M on windows to use multiprocess mode. Sandboxing is still disallowed, as gaol is not implemented on Windows. This depends on servo/ipc-channel#108 to provide windows support in ipc-channel. At the moment this causes a panic in ipc-channel when run with multiprocess (works fine single process), but I wanted to put this up so that others can test.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors

• There are tests for these changes OR
• These changes do not require tests because _____

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This change is

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/constellation/pipeline.rs

[jdm]

My preference would be to add a cfg block to the existing non-Windows code rather than duplicating the non-sandbox pieces here.

[vvuk]

The reason I didn't do that is that we don't have the gaol crate available on Windows at all. cfg!(...) evaluates to a boolean, so the types would still need to be available for both paths through the function. Is there a way to do this that I'm not seeing? The only other way I could think of was to create separate spawn_sandboxed() and spawn_unsandboxed() methods that spawn_multiprocess calls, and then have a windows-only spawn_sandboxed that panics.

With my latest changes at servo/ipc-channel#108, multiprocess Servo works on Windows.

[bors-servo]

☔ The latest upstream changes (presumably #14536) made this pull request unmergeable. Please resolve the merge conflicts.

[nox]

@jdm Are you ok with repeating these few lines of code for now? Should we land this?

[jdm]

We need servo/ipc-channel#108 to merge before this can. I'm ok with the code duplication.

[jdm]

No need to keep this open indefinitely.