The 'reason' was getting initialized to 'empty' state when not
provided through the constructor, which results in a crash when
accessed through throw_dom_exception_if_needed in the generated
IDL getter.
This fixes structured serialization of DataView. It was expected
to be uniform with TypedArray, which returns u32 for byte_offset().
This was covered by a number of WPT infrastructure tests, which this
commit also imports.
