Change the behavior of open on files to not truncate by default

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 8e4e3c14bcee9ddd4522ede070ca24165ff1745e..6d0522145d3c327f9267dc73188652b8c3781d05 100644
--- a/runtime/bin/file_win.cc
+++ b/runtime/bin/file_win.cc
@@ -100,10 +100,13 @@ off_t File::Length() {
 }
 
 
-File* File::Open(const char* name, bool writable) {
+File* File::Open(const char* name, FileOpenMode mode) {
   int flags = O_RDONLY | O_BINARY;
-  if (writable) {
-    flags = (O_RDWR | O_CREAT | O_TRUNC | O_BINARY);
+  if ((mode & kWrite) != 0) {
+    flags = (O_RDWR | O_CREAT | O_BINARY);
+  }
+  if ((mode & kTruncate) != 0) {
+    flags = flags | O_TRUNC;
   }
   int fd = open(name, flags, 0666);
   if (fd < 0) {