Fix clang style warnings in webrtc/modules/audio_coding

Mostly this consists of marking functions with override when
applicable, and moving function bodies from .h to .cc files.

BUG=163
R=henrik.lundin@webrtc.org, tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44979004

Cr-Commit-Position: refs/heads/master@{#8938}

webrtc/modules/audio_coding/BUILD.gn

@@ -51,12 +51,6 @@ source_set("audio_coding") {
     ":audio_coding_config",
   ]
 
-  if (is_clang) {
-    # Suppress warnings from Chrome's Clang plugins.
-    # See http://code.google.com/p/webrtc/issues/detail?id=163 for details.
-    configs -= [ "//build/config/clang:find_bad_constructs" ]
-  }
-
   if (is_win) {
     cflags = [
       # TODO(kjellander): Bug 261: fix this warning.