minidump: Use forward declarations in more places

minidump/minidump_exception_writer.h

Index: minidump/minidump_exception_writer.h
diff --git a/minidump/minidump_exception_writer.h b/minidump/minidump_exception_writer.h
index dc4ad6ce12d49508891843d6a719e0247deab3fe..b76b1ff11e66267717a329a3b0e14aa5961d99da 100644
--- a/minidump/minidump_exception_writer.h
+++ b/minidump/minidump_exception_writer.h
@@ -17,17 +17,16 @@
 
 #include <dbghelp.h>
 #include <stdint.h>
-#include <sys/types.h>
 
 #include <vector>
 
 #include "base/basictypes.h"
-#include "minidump/minidump_context_writer.h"
 #include "minidump/minidump_stream_writer.h"
-#include "util/file/file_writer.h"
 
 namespace crashpad {
 
+class MinidumpContextWriter;
+
 //! \brief The writer for a MINIDUMP_EXCEPTION_STREAM stream in a minidump file.
 class MinidumpExceptionWriter final : public internal::MinidumpStreamWriter {
  public: