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

net/disk_cache/blockfile/mapped_file.cc

Index: net/disk_cache/blockfile/mapped_file.cc
diff --git a/net/disk_cache/blockfile/mapped_file.cc b/net/disk_cache/blockfile/mapped_file.cc
index b2fedd2c787a418f244c90526b06c559f7911631..ae2bd327b2e82dfe37caf3ee60eda5a542d47411 100644
--- a/net/disk_cache/blockfile/mapped_file.cc
+++ b/net/disk_cache/blockfile/mapped_file.cc
@@ -23,6 +23,8 @@ bool MappedFile::Store(const FileBlock* block) {
 
 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;