This should fix (at least) the last two remaining WPT test failures in
FileAPI/unicode.html.
(cherry picked from commit 55f0dc823ff266aeb29fd6f7d47d5e7f89a50ee6)
Some steps are still to be implemented, namely:
* Properly aborting the read algorithm
* Handling BinaryString type properly
* Setting error on any error
But as it stands, this is enough functionality for the basic case of
reading the contents of a blob using the FileReader API.