Fix net::FileStream to handle all errors correctly.

Fix net::FileStream to handle all errors correctly. 

This CL adds FileStream::Context::IOResult struct that's used to pass 
results of IO operations inside FileStream::Context. Following bugs are 
fixes: 

1. On POSIX net::FileStream::Read() and net::FileStream::Write() would 
return a positive result in case of an error. This happens because 
POSIX errors are positive integers and FileStream was written with 
assumption that they are negative. 
2. On Windows Seek() and Flush() errors were not handled correctly. 
This is because CheckForIOError() was assuming that all error codes 
are negative, but RecordAndMapError() was mapping positive errors. 
3. On Windows results of asynchronous ReadFile() and WriteFile() were 
not handled correctly - ERR_IO_PENDING would be returned even when 
operation completes synchronously. 
4. On Windows OVERLAPPED struct wasn't set to zeros as necessary. 

Also added unittests to check that error codes are handled correctly 
now.

TBR=mmenke@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=184577

[Sergey Ulanov, 2013-02-26 01:49:14]

Third attempt to land https://codereview.chromium.org/12320003/ .
Verified that this version passes tests on XP now.

You were right that it's better to leave WriteSync() and ReadSync() behavior
changes for a separate CL. 

Apparently ReadFile() and WriteFile() ignore lpNumberOfBytesRead and
lpNumberOfBytesWritten for overlapped operations. GetOverlappedResult() should
always be used to get results of overlapped operations even when they complete
synchronously. The current code just leaves OVERLAPPED in signaled state so that
the async result is handled later.

[Sergey Ulanov, 2013-02-26 03:21:15]

Committed patchset #2 manually as r184577 (presubmit successful).