This CL introduces namespaces in the aec c++ files
(the ones that were recently moved from c)

There are many files changed but most changes just
consist of adding namespaces.

In aec_common.h an C++-specific #ifdef needed to be added as
that file is both included from C and C++. I could see no
way around that but please let me know if there is a better
way around that.

BUG=webrtc:5201

Review URL: https://codereview.webrtc.org/1766663002

Cr-Commit-Position: refs/heads/master@{#11883}
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index 2311c65..8af5f53 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -25,10 +25,10 @@
 #include "webrtc/modules/audio_processing/beamformer/array_util.h"
 #include "webrtc/typedefs.h"
 
-struct AecCore;
-
 namespace webrtc {
 
+struct AecCore;
+
 class AudioFrame;
 
 template<typename T>