build: move libcc patches to electron repo (#14104)

In the GN build, libchromiumcontent is no longer a distinct library, but
merely a container for a set of scripts and patches. Maintaining those
patches in a separate repository is tedious and error-prone, so merge
them into the main repo.

Once this is merged and GN is the default way to build Electron, the
libchromiumcontent repository can be archived.

patches/common/ffmpeg/build_gn.patch

@@ -0,0 +1,15 @@
+diff --git a/BUILD.gn b/BUILD.gn
+index 5439b39693..6783292c3b 100755
+--- a/BUILD.gn
++++ b/BUILD.gn
+@@ -393,6 +393,10 @@ if (is_component_ffmpeg) {
+     # So we can append below and assume they're defined.
+     ldflags = []
+ 
++    if (!is_component_build && is_mac) {
++      ldflags += [ "-Wl,-install_name,@rpath/libffmpeg.dylib" ]
++    }
++
+     if (is_fuchsia || (is_posix && !is_mac)) {
+       # Fixes warnings PIC relocation when building as component.
+       ldflags += [