Merge 159454 - Flush at the end of local file writing in FileWriter API.

Merge 159454 - Flush at the end of local file writing in FileWriter API.

This CL ensures the written content is flushed physically before
fileWriter.write() invokes the "onwriteend" event, for native local files.
Remote files (Google Drive files in Chrome OS) nor FileSystem API
files (in PERSISTENT or TEMPORARY storage) aren't affected.

The summary of the changes:
* Call Flush() before the final callback.
(webkit/fileapi/file_writer_delegate.cc)
* Delegate Flush() to net::FileStream::Flush
(webkit/fileapi/local_file_stream_writer.cc)
* No-op implementation for Flush().
(webkit/fileapi/sanbox_file_stream_writer.cc)
(webkit/chromeos/fileapi/remote_file_stream_writer.cc)
* Implementation of asynchronous Flush.
(net/base/file_stream_{posix,win}.cc)
* Other files are just for reflecting the rename Flush -> FlushSync.

BUG=144790
R=willchan@chromium.org,kinuko@chromium.org,benjhayden@chromium.org
TBR=marja@chromium.org
TEST=out/Debug/browser_tests --gtest_filter='*FileSystemApi*'
TEST=./webkit/tools/layout_tests/run_webkit_tests.sh fast/filesystem
TEST=Manual steps reported in the issue.

Review URL: https://codereview.chromium.org/10986045

TBR=kinaba@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=160297