Auto merge of #23696 - servo:hl, r=jdm

Add UWP port - bis

This is #23658 with some fixes:
- removed nspr from DLL dependencies (was breaking VS build)
- exclude symbols from header file (was breaking VS build)
- rebased

Before merging:
- please check the rebased commit that introduces the `--uwp` option (things moved around since 7c85dc09b5)
- should we wait until the WR fix lands upstream?

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components/servo/Cargo.toml

@@ -15,6 +15,7 @@ crate-type = ["rlib"]
 canvas2d-azure = ["canvas/canvas2d-azure"]
 canvas2d-raqote = ["canvas/canvas2d-raqote"]
 debugmozjs = ["script/debugmozjs"]
+egl = ["mozangle/egl"]
 energy-profiling = ["profile_traits/energy-profiling"]
 profilemozjs = ["script/profilemozjs"]
 googlevr = ["webvr/googlevr"]
@@ -85,4 +86,4 @@ git = "https://github.com/servo/media"
 git = "https://github.com/servo/media"
 
 [target.'cfg(target_os = "windows")'.dependencies]
-mozangle = { version = "0.2", features = ["egl"] }
+mozangle = {version = "0.2"}