[fsp] Add FileStreamReader for the reading operation.

This patch adds a FileStreamReader implementation for provided file systeme.
Note, that this is an initial version, which doesn't validate modification time.
That will be added in a separate patch.

TEST=browser_tests: *FileSystemProvider*ReadFile*,
     unit_tests: *FileSystemProvider*FileStreamReader*
BUG=248427
R=benwells@chromium.org, hirono@chromium.org, kinaba@chromium.org

Review URL: https://codereview.chromium.org/288113004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272041 0039d316-1c4b-4281-b951-d872f2087c98
21 files changed