Allow retrieval of media device ID salt even after ResourceContext has gone away.

content/test/webrtc_audio_device_test.cc

Index: content/test/webrtc_audio_device_test.cc
diff --git a/content/test/webrtc_audio_device_test.cc b/content/test/webrtc_audio_device_test.cc
index a57270317b803a7dccef625616e04ececf58475f..f28cd2d318ae80338c7fb5ebbb118c614d24a189 100644
--- a/content/test/webrtc_audio_device_test.cc
+++ b/content/test/webrtc_audio_device_test.cc
@@ -172,10 +172,6 @@ class MockRTCResourceContext : public ResourceContext {
     return false;
   }
 
-  virtual std::string GetMediaDeviceIDSalt() OVERRIDE {
-    return "";
-  }
-
  private:
   net::URLRequestContext* test_request_context_;