Add kPdfRenderer define for non-Windows platforms (see issue #3047)

This switch is currently only defined for Windows in
content/public/common/content_switches.h.

libcef/common/cef_switches.h

@@ -62,6 +62,10 @@ extern const char kFrameworkDirPath[];
 extern const char kMainBundlePath[];
 #endif
 
+#if !defined(OS_WIN)
+extern const char kPdfRenderer[];
+#endif
+
 }  // namespace switches
 
 #endif  // CEF_LIBCEF_COMMON_CEF_SWITCHES_H_