[fsp] First part of support for reading files.

This patch adds the read file operation, which is however not bound yet to
fileapi.

In the upcoming patch, support for FileStreamReader will be added. After that,
OpenFile, CloseFile and ReadFile will be bound to async file utils.

TEST=unit_tests: *FileSystemProvider*ReadFile*
BUG=248427

Review URL: https://codereview.chromium.org/287673004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270985 0039d316-1c4b-4281-b951-d872f2087c98
17 files changed