Add truncate operation to File API.

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 66ea550edc03f263e9ac8234c26dab8e062244e1..586a9e40bb57d55fc338f97be603d100ed4491f7 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -73,9 +73,15 @@ off_t File::Position() {
 }
 
 
-off_t File::SetPosition(int64_t position) {
+bool File::SetPosition(int64_t position) {
   ASSERT(handle_->fd() >= 0);
-  return lseek(handle_->fd(), position, SEEK_SET);
+  return (lseek(handle_->fd(), position, SEEK_SET) != -1);
+}
+
+
+bool File::Truncate(int64_t length) {
+  ASSERT(handle_->fd() >= 0);
+  return (ftruncate(handle_->fd(), length) != -1);
 }