Reland "Default to dlopening the PipeWire."

This is a reland of 0cc42d47389c039c57e47d7ec0c76b97e2da2b0b

Original change's description:
> Reland "Default to dlopening the PipeWire."
>
> This is a reland of a099877d8946eb942046ca1295cc142e4fa7ea6f
>
> Original change's description:
> > Reland "Default to dlopening the PipeWire."
> >
> > This is a reland of a13be019017449c57f48203d0fb778f34f7553a7
> >
> > Original change's description:
> > > Default to dlopening the PipeWire.
> > >
> > > Reuse the existing infra from Chromium to do that. Additionally the
> > > target_gen_dir needs to the added to the include directories, otherwise
> > > the Chromium build will fail as it won't find the generated stubs. Also the
> > > pw_properties_new() was replaced with pw_properties_new_string() as it doesn't
> > > require a variadic parameter because the //tools/generate_stubs/generate_stubs.py
> > > doesn't work with them correctly. With all these changes in place the PipeWire
> > > support is enabled when compiling on Linux.
> > >
> > > Bug: chromium:682122
> > > Change-Id: I3bbc5efaecd9a08e20cbcf998b2cb534224eae7d
> > > Reviewed-on: https://webrtc-review.googlesource.com/c/111081
> > > Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> > > Reviewed-by: Brave Yao <braveyao@webrtc.org>
> > > Commit-Queue: Tomáš Popela <tomas.popela@gmail.com>
> > > Cr-Commit-Position: refs/heads/master@{#25720}
> >
> > Bug: chromium:682122
> > Change-Id: I3cca3d4d961dc7a088346c8fd3c970d3dfde3b79
> > Reviewed-on: https://webrtc-review.googlesource.com/c/113040
> > Reviewed-by: Weiyong Yao <braveyao@chromium.org>
> > Reviewed-by: Brave Yao <braveyao@webrtc.org>
> > Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> > Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
> > Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#25981}
>
> Bug: chromium:682122
> Change-Id: Ief26c93069f946f981340664a267fcb412229285
> Reviewed-on: https://webrtc-review.googlesource.com/c/114163
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Brave Yao <braveyao@webrtc.org>
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#26004}

Bug: chromium:682122
Change-Id: I0a4ea7b39be5970f26df6dbc3e437dd63cdb8708
Reviewed-on: https://webrtc-review.googlesource.com/c/116280
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Brave Yao <braveyao@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26154}
diff --git a/BUILD.gn b/BUILD.gn
index c9249ff..4db6558 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -131,6 +131,14 @@
       # Allow includes to be prefixed with webrtc/ in case it is not an
       # immediate subdirectory of the top-level.
       ".",
+
+      # Just like the root WebRTC directory is added to include path, the
+      # corresponding directory tree with generated files needs to be added too.
+      # Note: this path does not change depending on the current target, e.g.
+      # it is always "//gen/third_party/webrtc" when building with Chromium.
+      # See also: http://cs.chromium.org/?q=%5C"default_include_dirs
+      # https://gn.googlesource.com/gn/+/master/docs/reference.md#target_gen_dir
+      target_gen_dir,
     ]
   }
   if (is_posix || is_fuchsia) {