Moved the AEC C code to be built using C++.

In order for the change to be reviewable, the
move was made into two steps consisting of the
first two patches in this CL.

Step 1 (patch set 1):
-Changed file types to use .cc
-Changed buildfiles to use the new files
-Changed C code inclusion to properly match the changed
 file formats (removed and added extern "C" declarations).
-Changed implicit void-> nonvoid casts that are
 illegal in C++ to be explicit.

Step 2 (patch set 2):
-Changed all the warnings reported when uploading the CL.
-The warnings about formatting of the assembly optimized
 code were not addressed though.

BUG=webrtc:5201

Review URL: https://codereview.webrtc.org/1713923002

Cr-Commit-Position: refs/heads/master@{#11727}

webrtc/modules/audio_processing/audio_processing_impl.cc

@@ -20,9 +20,7 @@
 #include "webrtc/common_audio/channel_buffer.h"
 #include "webrtc/common_audio/include/audio_util.h"
 #include "webrtc/common_audio/signal_processing/include/signal_processing_library.h"
-extern "C" {
 #include "webrtc/modules/audio_processing/aec/aec_core.h"
-}
 #include "webrtc/modules/audio_processing/agc/agc_manager_direct.h"
 #include "webrtc/modules/audio_processing/audio_buffer.h"
 #include "webrtc/modules/audio_processing/beamformer/nonlinear_beamformer.h"