Build fix

git-svn-id: svn://svn.chromium.org/blink/trunk@42684 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/WebKit/mac/WebKitPrefix.h

@@ -97,3 +97,6 @@ typedef float CGFloat;
 /* Work around bug with C++ library that screws up Objective-C++ when exception support is disabled. */
 #undef try
 #undef catch
+
+#define JS_EXPORTDATA
+#define WEBKIT_EXPORTDATA