Bug: OS::MemoryMappedFile::open() should not truncate a pre-existing file.

src/platform-win32.cc

Index: src/platform-win32.cc
===================================================================
--- src/platform-win32.cc	(revision 6858)
+++ src/platform-win32.cc	(working copy)
@@ -938,7 +938,7 @@
 OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name) {
   // Open a physical file
   HANDLE file = CreateFileA(name, GENERIC_READ | GENERIC_WRITE,
-      FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
+      FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_ALWAYS, 0, NULL);
   if (file == NULL) return NULL;
 
   int size = static_cast<int>(GetFileSize(file, NULL));