Auto merge of #17425 - paulrouget:attach-pipeline-2, r=asajeffrey

cleanup embedder/compositor/constellation/script messages

Fix: #17226 #17200 #17201

This is work in progress. Some tests still fail.
I'd like to get early feedback as it's a pretty large PR.

There is nothing fundamentally new. Basically, I added TopLevelBrowsingContrextId to the relevant messages between the embedder, the compositor and the constellation, and enforced the PipelineId to be attached to each ScriptMsg (see #17201).

I unaliased all the ScriptMsg. It was getting difficult to understand the nature of the message as ScriptMsg was used aliased CompositorMsg sometimes (CompositorMsg is an actually type of message already). I renamed constellation_chan to script_to_constellation_chan, again, for clarification.

This cleanup code is necessary for #15934 and for tabs support.

/cc @asajeffrey can I ask you to look at this? No need for a formal review, I need feedback at this stage.

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