OFStream should have a virtual destructor.

I noticed that using --print-opt-code was producing output unreliably.
Upon investigation, it turns out OFStream's destructor was not marked
virtual. Fixing this fixes the problem, as ostream flushes at destruction.

BUG=

Review URL: https://codereview.chromium.org/1503383003

Cr-Commit-Position: refs/heads/master@{#32688}

src/ostreams.h

@@ -36,7 +36,7 @@ class OFStreamBase : public std::streambuf {
 class OFStream : public std::ostream {
  public:
   explicit OFStream(FILE* f);
-  ~OFStream();
+  virtual ~OFStream();
 
  private:
   OFStreamBase buf_;