Move --user-agent overriding logic from chrome into content.

content/test/test_content_client.cc

Index: content/test/test_content_client.cc
diff --git a/content/test/test_content_client.cc b/content/test/test_content_client.cc
index 455f2e95de84387e16b9668d68e3fabebb5c8d4c..c62e662650bdcad6ebadd0714dc67855b71cb3a4 100644
--- a/content/test/test_content_client.cc
+++ b/content/test/test_content_client.cc
@@ -44,8 +44,7 @@ bool TestContentClient::CanHandleWhileSwappedOut(const IPC::Message& msg) {
   return false;
 }
 
-std::string TestContentClient::GetUserAgent(bool* overriding) const {
-  *overriding = false;
+std::string TestContentClient::GetUserAgent() const {
   return std::string("TestContentClient");
 }