Bug 1446900: Enable bug 1348361 (async process launching) on nightly only. r=ted

dom/ipc/moz.build

@@ -175,3 +175,6 @@ CXXFLAGS += CONFIG['TK_CFLAGS']
 
 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     CXXFLAGS += ['-Wno-error=shadow']
+
+if CONFIG['NIGHTLY_BUILD']:
+    DEFINES['ASYNC_CONTENTPROC_LAUNCH'] = True

ipc/glue/moz.build

@@ -228,3 +228,6 @@ if CONFIG['MOZ_SANDBOX'] and CONFIG['OS_ARCH'] == 'WINNT':
 
 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     CXXFLAGS += ['-Wno-shadow']
+
+if CONFIG['NIGHTLY_BUILD']:
+    DEFINES['ASYNC_CONTENTPROC_LAUNCH'] = True