Move AsyncFileSystem.h and FileSystemType.h to platform

FileSystemType is exposed via the public platform API, which means we should
probably move it into Source/platform.

BUG=297477
R=eseidel

Review URL: https://codereview.chromium.org/26548003

git-svn-id: svn://svn.chromium.org/blink/trunk@159166 bbb929c8-8fbe-4397-9dbb-9b2b20218538