[PPAPI] It is now possible to pass filesystems from JavaScript to NaCl modules.

If a DOMFileSystem is passed as a message to the NaCl module, it will be
converted into a resource var which is available to the plugin via the dev
interface PPB_VarResource_Dev.

BUG=177017

Review URL: https://codereview.chromium.org/26564009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232080 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232080