Issue #80 - reinstated unified building for some large chunks of our code.

This should reduce compile complexity saving time and reducing linker stress.

dom/media/webrtc/moz.build

@@ -30,14 +30,15 @@ if CONFIG['MOZ_WEBRTC']:
     UNIFIED_SOURCES += [
         'MediaEngineCameraVideoSource.cpp',
         'MediaEngineRemoteVideoSource.cpp',
-    ]
-    SOURCES += [
         'MediaEngineTabVideoSource.cpp',
-        'MediaEngineWebRTC.cpp',
         'MediaEngineWebRTCAudio.cpp',
         'RTCCertificate.cpp',
         'RTCIdentityProviderRegistrar.cpp',
     ]
+    # MediaEngineWebRTC.cpp needs to be built separately.
+    SOURCES += [
+        'MediaEngineWebRTC.cpp',
+    ]
     LOCAL_INCLUDES += [
         '/dom/base',
         '/media/libyuv/include',
@@ -51,10 +52,8 @@ XPIDL_SOURCES += [
 ]
 
 UNIFIED_SOURCES += [
-    'MediaTrackConstraints.cpp',
-]
-SOURCES += [
     'MediaEngineDefault.cpp',
+    'MediaTrackConstraints.cpp',
     'PeerIdentity.cpp',
 ]