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.

build/config/BUILD.gn

@@ -0,0 +1,25 @@
+config("build_time_executable") {
+  configs = []
+
+  if (is_electron_build && !is_component_build) {
+    # The executables which have this config applied are dependent on ffmpeg,
+    # which is always a shared library in an Electron build. However, in the
+    # non-component build, executables don't have rpath set to search for
+    # libraries in the executable's directory, so ffmpeg cannot be found. So
+    # let's make sure rpath is set here.
+    # See '//build/config/gcc/BUILD.gn' for details on the rpath setting.
+    if (is_linux) {
+      configs += [ "//build/config/gcc:rpath_for_built_shared_libraries" ]
+    }
+
+    if (is_mac) {
+      ldflags = [ "-Wl,-rpath,@loader_path/." ]
+    }
+  }
+}
+# For MAS build, we force defining "MAS_BUILD".
+config("mas_build") {
+  if (is_mas_build) {
+    defines = [ "MAS_BUILD" ]
+  }
+}