Add a SetDoNotFragment() method to DatagramSocket, and call this for

blimp/net/test_common.h

Index: blimp/net/test_common.h
diff --git a/blimp/net/test_common.h b/blimp/net/test_common.h
index c726feba5a8c2645cf72d5f2d4e59f95fba64786..da158baf5c35ac525c1a77c88c74709cf35782ee 100644
--- a/blimp/net/test_common.h
+++ b/blimp/net/test_common.h
@@ -111,6 +111,7 @@ class MockStreamSocket : public net::StreamSocket {
   MOCK_METHOD3(Write, int(net::IOBuffer*, int, const net::CompletionCallback&));
   MOCK_METHOD1(SetReceiveBufferSize, int(int32_t));
   MOCK_METHOD1(SetSendBufferSize, int(int32_t));
+  MOCK_METHOD1(SetDoNotFragment, int(bool));
   MOCK_METHOD1(Connect, int(const net::CompletionCallback&));
   MOCK_METHOD0(Disconnect, void());
   MOCK_CONST_METHOD0(IsConnected, bool());