Implement postMessage for ServiceWorkers
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Fixes#12773
r? @jdm
Changes:
* Implements `postMessage` on `ServiceWorker` object.
* Removes unused channels from sw and their scopes.
* Fixes a crash when calling `scope.script_chan()` in sw-scopes event handling.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#12773
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- [X] There are tests for these changes at `tests/html/service-worker`
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Consistently use the name 'pipeline_id' to refer to a function that
returns an (optional) PipelineId.
This was prompted by discovering both fn pipeline and fn pipeline_id
doing the same job in htmliframeelement.rs.
Note that there is fn pipeline in components/compositing/compositor.rs,
but that actually returns an Option<&CompositionPipeline>, not any kind
of PipelineId.
