Implement setPosition on File to seek to a byte position.

runtime/bin/file_linux.cc

Index: runtime/bin/file_linux.cc
diff --git a/runtime/bin/file_linux.cc b/runtime/bin/file_linux.cc
index 6da08185133bf283e98961d96f324d8a2b1ebf8b..b816cc7e7a53430afe91bb0d2c45480f19e4f3dc 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -72,6 +72,12 @@ 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());