Support length operation without opening file.

runtime/bin/file_win.cc

Index: runtime/bin/file_win.cc
diff --git a/runtime/bin/file_win.cc b/runtime/bin/file_win.cc
index e5f7206b5dc5799576335c233611fd8630ec53a4..1293dce802ec5f57ccdb8b1cc2ff713cfa9f70b5 100644
--- a/runtime/bin/file_win.cc
+++ b/runtime/bin/file_win.cc
@@ -90,14 +90,11 @@ bool File::Flush() {
 
 off_t File::Length() {
   ASSERT(handle_->fd() >= 0);
-  off_t position = lseek(handle_->fd(), 0, SEEK_CUR);
-  if (position < 0) {
-    // The file is not capable of seeking. Return an error.
-    return -1;
+  struct stat st;
+  if (fstat(handle_->fd(), &st) == 0) {
+    return st.st_size;
   }
-  off_t result = lseek(handle_->fd(), 0, SEEK_END);
-  lseek(handle_->fd(), position, SEEK_SET);
-  return result;
+  return -1;
 }
 
 
@@ -157,6 +154,15 @@ bool File::Delete(const char* name) {
 }
 
 
+off_t File::LengthFromName(const char* name) {
+  struct stat st;
+  if (stat(name, &st) == 0) {
+    return st.st_size;
+  }
+  return -1;
+}
+
+
 bool File::IsAbsolutePath(const char* pathname) {
   // Should we consider network paths?
   if (pathname == NULL) return false;