FileReader: switch to using TypeChecking=Interface 

Let generated bindings code handle the type checking of Blob arguments.

R=tzik@chromium.org
BUG=

Review URL: https://codereview.chromium.org/474353004

git-svn-id: svn://svn.chromium.org/blink/trunk@180454 bbb929c8-8fbe-4397-9dbb-9b2b20218538