build: [gn] centralize GN args under build/args/*.gn

Devs can still locally override or add settings by adding lines after
the `import` statement in args.gn.

This will help make sure that everyone has a common set of build args,
and will allow us to update everyone's build args in git.

build/args/all.gn

@@ -0,0 +1,10 @@
+is_electron_build = true
+is_electron_gn_build = true
+root_extra_deps = [ "//electron" ]
+
+v8_promise_internal_field_count = 1
+v8_typed_array_max_size_in_heap = 0
+
+enable_widevine = true
+proprietary_codecs = true
+ffmpeg_branding = "Chrome"