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

base/files/memory_mapped_file.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/files/memory_mapped_file.h"
 
 #include <utility>
 
 #include "base/files/file_path.h"
 #include "base/logging.h"
@@ skipping 53 matching lines @@
 bool MemoryMappedFile::Initialize(File file, Access access) {
   DCHECK_NE(READ_WRITE_EXTEND, access);
   return Initialize(std::move(file), Region::kWholeFile, access);
 }
 
 bool MemoryMappedFile::Initialize(File file,
                                   const Region& region,
                                   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.";
         return false;
       }
       // Fall through.
     case READ_ONLY:
     case READ_WRITE:
       // Ensure that the region values are valid.
       if (region.offset < 0 || region.size < 0) {
@@ skipping 35 matching lines @@
   const int64_t mask =
       static_cast<int64_t>(SysInfo::VMAllocationGranularity()) - 1;
   DCHECK_LT(mask, std::numeric_limits<int32_t>::max());
   *offset = start & mask;
   *aligned_start = start & ~mask;
   *aligned_size = (size + *offset + mask) & ~mask;
 }
 #endif
 
 }  // namespace base