Revert of Added new mixer to |check_targets| in .gn and fixed include/depend errors. (patchset #1 id:1 of https://codereview.webrtc.org/2234293002/ )

Reason for revert:
Breaks Chromium FYI builds, for example https://build.chromium.org/p/chromium.webrtc.fyi/builders/Linux%20Builder/builds/9219/steps/compile/logs/stdio

Original issue's description:
> Added new mixer to |check_targets| in .gn and fixed include/depend errors.
>
> Also fixed one small chromium-style error in the new mixer.
>
> NOTRY=True
>
> Committed: https://crrev.com/d700bef583d29ba2834ae57b3af7e8d3b8306cb9
> Cr-Commit-Position: refs/heads/master@{#13752}

TBR=kjellander@webrtc.org,aleloi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.webrtc.org/2241243002
Cr-Commit-Position: refs/heads/master@{#13753}
diff --git a/.gn b/.gn
index 8a173ba..e151f7c 100644
--- a/.gn
+++ b/.gn
@@ -19,10 +19,7 @@
 # their includes checked for proper dependencies when you run either
 # "gn check" or "gn gen --check".
 # TODO(kjellander): Keep adding paths to this list as work in webrtc:5589 is done.
-check_targets = [
-  "//webrtc/voice_engine:level_indicator",
-  "//webrtc/modules/audio_mixer:audio_conference_mixer",
-]
+check_targets = [ "//webrtc/voice_engine:level_indicator" ]
 
 # These are the list of GN files that run exec_script. This whitelist exists
 # to force additional review for new uses of exec_script, which is strongly
diff --git a/webrtc/modules/audio_mixer/BUILD.gn b/webrtc/modules/audio_mixer/BUILD.gn
index 2449c54..7ef2790 100644
--- a/webrtc/modules/audio_mixer/BUILD.gn
+++ b/webrtc/modules/audio_mixer/BUILD.gn
@@ -72,11 +72,9 @@
   }
 
   deps = [
-    "../..:webrtc_common",
     "../../base:rtc_base_approved",
     "../../modules/audio_processing",
     "../../modules/utility",
     "../../system_wrappers",
-    "../../system_wrappers:system_wrappers_default",
   ]
 }
diff --git a/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.cc b/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.cc
index 48380fc..5be76ec 100644
--- a/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.cc
+++ b/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.cc
@@ -19,6 +19,7 @@
 #include "webrtc/modules/utility/include/audio_frame_operations.h"
 #include "webrtc/system_wrappers/include/critical_section_wrapper.h"
 #include "webrtc/system_wrappers/include/trace.h"
+#include "webrtc/voice_engine/utility.h"
 
 namespace webrtc {
 namespace {
@@ -141,8 +142,6 @@
   thread_checker_.DetachFromThread();
 }
 
-NewAudioConferenceMixerImpl::~NewAudioConferenceMixerImpl() {}
-
 bool NewAudioConferenceMixerImpl::Init() {
   crit_.reset(CriticalSectionWrapper::CreateCriticalSection());
   if (crit_.get() == NULL)
diff --git a/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.h b/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.h
index 3933350..ef93e6f 100644
--- a/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.h
+++ b/webrtc/modules/audio_mixer/new_audio_conference_mixer_impl.h
@@ -19,6 +19,7 @@
 #include "webrtc/base/thread_checker.h"
 #include "webrtc/engine_configurations.h"
 #include "webrtc/modules/audio_mixer/new_audio_conference_mixer.h"
+#include "webrtc/modules/audio_conference_mixer/source/memory_pool.h"
 #include "webrtc/modules/include/module_common_types.h"
 
 namespace webrtc {
@@ -63,8 +64,6 @@
 
   explicit NewAudioConferenceMixerImpl(int id);
 
-  ~NewAudioConferenceMixerImpl() override;
-
   // Must be called after ctor.
   bool Init();