Fix renderer crash in Blob constructor.

The CL that made the Blob constructor accept WebIDL sequences (according
to the File API spec) did not handle the case of a sequence-like object
that throws an error on a property access. This CL fixes the oversight.

BUG=319258

Review URL: https://codereview.chromium.org/61733022

git-svn-id: svn://svn.chromium.org/blink/trunk@162035 bbb929c8-8fbe-4397-9dbb-9b2b20218538