Add AudioOption for residual echo detector, and enable the echo detector by default on non-mobile platforms.

BUG=webrtc:6525

Review-Url: https://codereview.webrtc.org/2493753002
Cr-Commit-Position: refs/heads/master@{#15079}
diff --git a/webrtc/modules/audio_processing/include/audio_processing.h b/webrtc/modules/audio_processing/include/audio_processing.h
index c899e80..2c0f554 100644
--- a/webrtc/modules/audio_processing/include/audio_processing.h
+++ b/webrtc/modules/audio_processing/include/audio_processing.h
@@ -259,7 +259,11 @@
       float initial_peak_level_dbfs = -6.0206f;
     } level_controller;
     struct ResidualEchoDetector {
+#if defined(WEBRTC_ANDROID) || defined(WEBRTC_IOS)
       bool enabled = false;
+#else
+      bool enabled = true;
+#endif
     } residual_echo_detector;
   };