Fix error handling in POSIX version of the base::File::GetLength.

net/disk_cache/blockfile/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 "net/disk_cache/blockfile/mapped_file.h"
 
 #include <algorithm>
 #include <memory>
 
 namespace disk_cache {
 
 // Note: Most of this class is implemented in platform-specific files.
 
 bool MappedFile::Load(const FileBlock* block) {
   size_t offset = block->offset() + view_size_;
   return Read(block->buffer(), block->size(), offset);
 }
 
 bool MappedFile::Store(const FileBlock* block) {
   size_t offset = block->offset() + view_size_;
   return Write(block->buffer(), block->size(), offset);
 }
 
 bool MappedFile::Preload() {
   size_t file_len = GetLength();
+  if (!file_len)

[danakj, 2016/11/29 01:48:01]

I don't think this is strictly needed. Reading 0 is okay, and would have
returned true before? We shouldn't change any behaviour in this CL.

+    return false;
   std::unique_ptr<char[]> buf(new char[file_len]);
   if (!Read(buf.get(), file_len, 0))
     return false;
   return true;
 }
 }  // namespace disk_cache