Refactor webrtc/modules/media_file for GN check

This moves some GN check configurations out of .gn to individual
targets.

The now checked target is:
"//webrtc/modules/media_file/*"

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2593693002
Cr-Commit-Position: refs/heads/master@{#15726}

.gn

@@ -29,6 +29,7 @@ check_targets = [
   "//webrtc/modules/audio_mixer/*",
   "//webrtc/modules/audio_processing/*",
   "//webrtc/modules/desktop_capture/*",
+  "//webrtc/modules/media_file/*",
   "//webrtc/modules/video_capture/*",
   "//webrtc/modules/video_coding/*",
   "//webrtc/stats:rtc_stats",

webrtc/modules/media_file/BUILD.gn

@@ -34,6 +34,7 @@ rtc_static_library("media_file") {
 
   deps = [
     "../..:webrtc_common",
+    "../../base:rtc_base_approved",
     "../../common_audio",
     "../../system_wrappers",
   ]