Auto merge of #19039 - sylvestre:master, r=emilio

Support multi lines declarations in the parsing of the Fx header files

For example, in dom/base/nsGkAtomList.h, we currently have:

`GK_ATOM(mouseWheel, "mouseWheel")  // For discrete wheel events (e.g. not OSX magic mouse)`

but if we change to
```
GK_ATOM(mouseWheel,
        "mouseWheel")  // For discrete wheel events (e.g. not OSX magic mouse)
```
The parser didn't handle the declaration

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