Fix NetEq performance test regression

The test code created an AudioBuffer object inside the work loop. This
turned out to be expensive, since the AudioBuffer ctor implicitly
called memset on all of the audio data array. The obvious remedy is to
create the buffer outside of the loop. This does not have any impact
apart from the performance boost, since the output data from NetEq is
not even considered in the test.

BUG=chromium:592907,webrtc:5647
TBR=ivoc@webrtc.org
NOTRY=true

Review URL: https://codereview.webrtc.org/1782803002

Cr-Commit-Position: refs/heads/master@{#11940}

webrtc/modules/include/module_common_types.h

@@ -541,6 +541,8 @@ class AudioFrame {
   RTC_DISALLOW_COPY_AND_ASSIGN(AudioFrame);
 };
 
+// TODO(henrik.lundin) Can we remove the call to data_()?
+// See https://bugs.chromium.org/p/webrtc/issues/detail?id=5647.
 inline AudioFrame::AudioFrame()
     : data_() {
   Reset();