Tidy ELF builder.

Don't do "if (ptr)". Use const. Use DISALLOW_COPY_AND_ASSIGN. Avoid public
member variables.
Move ValueObject to base and use in ELF builder.
Tidy VectorOutputStream to not use non-const reference arguments.

Change-Id: I2c727c3fc61769c3726de7cfb68b2d6eb4477e53
diff --git a/compiler/output_stream_test.cc b/compiler/output_stream_test.cc
index 315ca09..bba9892 100644
--- a/compiler/output_stream_test.cc
+++ b/compiler/output_stream_test.cc
@@ -90,7 +90,7 @@
 
 TEST_F(OutputStreamTest, Vector) {
   std::vector<uint8_t> output;
-  VectorOutputStream output_stream("test vector output", output);
+  VectorOutputStream output_stream("test vector output", &output);
   SetOutputStream(output_stream);
   GenerateTestOutput();
   CheckTestOutput(output);