Don't realize existing parts of an extended memory-mapped file.

base/files/memory_mapped_file.cc

Index: base/files/memory_mapped_file.cc
diff --git a/base/files/memory_mapped_file.cc b/base/files/memory_mapped_file.cc
index 67890d68855cb810a086da788d2f046f78240d25..45a09a01aeb97eb3e06e84ac98253af83d4df0c1 100644
--- a/base/files/memory_mapped_file.cc
+++ b/base/files/memory_mapped_file.cc
@@ -71,6 +71,7 @@ bool MemoryMappedFile::Initialize(File file,
                                   Access access) {
   switch (access) {
     case READ_WRITE_EXTEND:
+      DCHECK(Region::kWholeFile != region);
       // Ensure that the extended size is within limits of File.
       if (region.size > std::numeric_limits<int64_t>::max() - region.offset) {
         DLOG(ERROR) << "Region bounds exceed maximum for base::File.";