[fsp] Add support for reading directories.

This patch adds support to list contents of directories.

TBR=isherman@chromium.org
TEST=unit_tests, browser_test: *FileSystemProvider*ReadDirectory
BUG=248427

Review URL: https://codereview.chromium.org/246913003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269201 0039d316-1c4b-4281-b951-d872f2087c98
24 files changed