Changing to using factory methods for some classes in NetEq

In this CL, the Expand, Accelerate and PreemptiveExpand objects are
created using factory methods. The factory methods are injected into
NetEqImpl on creation. This is a step towards implementing a no-decode
operation.

BUG=2776
R=turaj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/6999005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5382 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/modules/audio_coding/neteq4/preemptive_expand.cc b/webrtc/modules/audio_coding/neteq4/preemptive_expand.cc
index ac787eb..c7ce310 100644
--- a/webrtc/modules/audio_coding/neteq4/preemptive_expand.cc
+++ b/webrtc/modules/audio_coding/neteq4/preemptive_expand.cc
@@ -98,4 +98,11 @@
   }
 }
 
+PreemptiveExpand* PreemptiveExpandFactory::Create(
+    int sample_rate_hz,
+    size_t num_channels,
+    const BackgroundNoise& background_noise) const {
+  return new PreemptiveExpand(sample_rate_hz, num_channels, background_noise);
+}
+
 }  // namespace webrtc