Bug 940153 - Build dom/encoding in unified mode. r=bzbarsky

dom/encoding/moz.build

@@ -16,6 +16,9 @@ EXPORTS.mozilla.dom += [
 SOURCES += [
     'EncodingUtils.cpp',
     'FallbackEncoding.cpp',
+]
+
+UNIFIED_SOURCES += {
     'TextDecoder.cpp',
     'TextEncoder.cpp',
 ]