Apply TypeChecking to MediaSource IDLs

This allows cleaning up a number of null-checks, since the generated IDL
code now assures non-null where indicated in the IDL.
Also drop explicit throwTypeError() in MediaSourceBase::endOfStream, since
the bindings code would've thrown that already.
Add 'unrestricted' to the IDLs where the spec has them.

This updates to match spec. revision ba5233c6654d.
https://dvcs.w3.org/hg/html-media/rev/ba5233c6654d
...per W3 bug:
Bug 25347 - [MSE] 'nullable' arguments and type conversions and
INVALID_ACCESS_ERR (vs. WebIDL)
https://www.w3.org/Bugs/Public/show_bug.cgi?id=25347

One additional non-specced change is to mark arguments to
SourceBuffer.remove() as 'unrestricted' since that's the
semantics they expect.

BUG=368634
BUG=354298

Review URL: https://codereview.chromium.org/266453005

git-svn-id: svn://svn.chromium.org/blink/trunk@173197 bbb929c8-8fbe-4397-9dbb-9b2b20218538