[Cronet] Export NSError constants instead of defining them in Cronet.h

components/cronet/ios/Cronet.mm

Index: components/cronet/ios/Cronet.mm
diff --git a/components/cronet/ios/Cronet.mm b/components/cronet/ios/Cronet.mm
index 489b70c3a1d53ea425d88d94682f753b523930a7..9c827d2c936cc2fcc0cf0bb636bba44e3973ca24 100644
--- a/components/cronet/ios/Cronet.mm
+++ b/components/cronet/ios/Cronet.mm
@@ -22,6 +22,10 @@
 #include "net/cert/cert_verifier.h"
 #include "net/url_request/url_request_context_getter.h"
 
+// Cronet NSError constants.
+NSString* const CRNCronetErrorDomain = @"CRNCronetErrorDomain";
+NSString* const CRNInvalidArgumentKey = @"CRNInvalidArgumentKey";
+
 namespace {
 
 class CronetHttpProtocolHandlerDelegate;