build: [gn] explicitly override rtc_use_h264 (#14378)

* build: [gn] explicitly override rtc_use_h264

* build: [gn] don't override ffmpeg config from release config

* build: [gn] remove is_cfi override from release config

The original concern about node is fixed and we have been
building testing builds without the override for sometime now.

build/args/debug.gn

@@ -1,3 +1,10 @@
 import("all.gn")
 is_debug = true
 is_component_build = true
+
+# This may be guarded behind is_chrome_branded alongside
+# proprietary_codecs https://webrtc-review.googlesource.com/c/src/+/36321,
+# explicitly override here to build OpenH264 encoder/FFmpeg decoder.
+# The initialization of the decoder depends on whether ffmpeg has
+# been built with H.264 support.
+rtc_use_h264 = proprietary_codecs