[webkit glue] Make signedPublicKeyAndChallengeString() return an empty instead of null string.

This is needed for the <keygen> regression test to work

The webkit bug (and associated discussion) is here:
https://bugs.webkit.org/show_bug.cgi?id=70617

BUG=chromium-os:21905
TEST=regression test now works without additional changes to appendForm()

Change-Id: I0fffb6a885a75ac7a0acecce06034d8cdeab6832


Review URL: http://codereview.chromium.org/8401019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@107906 0039d316-1c4b-4281-b951-d872f2087c98

webkit/glue/webkitplatformsupport_impl.cc

@@ -595,8 +595,7 @@ WebKit::WebString WebKitPlatformSupportImpl::signedPublicKeyAndChallengeString(
     unsigned key_size_index,
     const WebKit::WebString& challenge,
     const WebKit::WebURL& url) {
-  NOTREACHED();
-  return WebKit::WebString();
+  return WebKit::WebString("");
 }
 
 #if defined(OS_LINUX)