Implement setPosition on File to seek to a byte position.

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 8f5c747c26a35b32a978e52017ab836787b3ba83..3cc3911fc7ceb96f5251d1a5401f06b8fd71c64e 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -73,6 +73,12 @@ off_t File::Position() {
 }
 
 
+off_t File::SetPosition(int64_t position) {
+  ASSERT(handle_->fd() >= 0);
+  return lseek(handle_->fd(), position, SEEK_SET);
+}
+
+
 void File::Flush() {
   ASSERT(handle_->fd() >= 0);
   fsync(handle_->fd());