modules/audio_coding: Remove some codec include dirs

Also clean up some include_dir entries and update the few
references to them with absolute include paths instead.
Finally fixed a few lint errors and invalid header guards.

None of these are used downstream.

BUG=webrtc:5095
TESTED=git cl try -c --bot=android_compile_rel --bot=linux_compile_rel --bot=win_compile_rel --bot=mac_compile_rel --bot=ios_rel --bot=linux_gn_rel --bot=win_x64_gn_rel --bot=mac_x64_gn_rel --bot=android_gn_rel -m tryserver.webrtc
R=kwiberg@webrtc.org

Review URL: https://codereview.webrtc.org/1438663003 .

Cr-Commit-Position: refs/heads/master@{#10700}
diff --git a/webrtc/modules/audio_coding/codecs/opus/opus.gypi b/webrtc/modules/audio_coding/codecs/opus/opus.gypi
index 05da3e5..d7454d6 100644
--- a/webrtc/modules/audio_coding/codecs/opus/opus.gypi
+++ b/webrtc/modules/audio_coding/codecs/opus/opus.gypi
@@ -39,17 +39,14 @@
       'dependencies': [
         'audio_encoder_interface',
       ],
-      'include_dirs': [
-        '<(webrtc_root)',
-      ],
       'sources': [
         'audio_decoder_opus.cc',
+        'audio_decoder_opus.h',
         'audio_encoder_opus.cc',
-        'include/audio_decoder_opus.h',
-        'include/audio_encoder_opus.h',
-        'include/opus_interface.h',
+        'audio_encoder_opus.h',
         'opus_inst.h',
         'opus_interface.c',
+        'opus_interface.h',
       ],
     },
   ],
@@ -65,9 +62,6 @@
             '<(webrtc_root)/test/test.gyp:test_support_main',
             '<(DEPTH)/testing/gtest.gyp:gtest',
           ],
-          'include_dirs': [
-            '<(webrtc_root)',
-          ],
           'sources': [
             'opus_fec_test.cc',
           ],