Bug 1610610 part 3. Fix the exception thrown when a SourceBuffer's mode is set to "segments". r=jya

I tried to write a test for this, but it looks like
mCurrentAttributes.mGenerateTimestamps is only true when the type is "audio/aac"
or "audio/mpeg", and we don't support either of those in MediaSource. So this
code can't actually be reached, as far as I can tell.

Differential Revision: https://phabricator.services.mozilla.com/D60597