Sync Blob.close() behavior wrt updated spec.

The File API now defines Blobs as having a readability state of
opened/closed, http://dev.w3.org/2006/webapi/FileAPI/#readabilityState
With close() naturally making the Blob enter the 'closed' state,

  http://dev.w3.org/2006/webapi/FileAPI/#close-method

Like the FileReader operations, Blob methods will now throw
InvalidStateError when passed a Blob in such a 'closed' state.

Update the implementation to follow the spec.

R=kinuko@chromium.org
BUG=344820

Review URL: https://codereview.chromium.org/184473002

git-svn-id: svn://svn.chromium.org/blink/trunk@168567 bbb929c8-8fbe-4397-9dbb-9b2b20218538