Move HTMLTable*Element.insert{Row,Cell}'s argument default into IDL file

They each take an index argument, that should default to -1.

This matches the specification, and doesn't change behavior since these
defaults were already implemented in the C++ code.

BUG=258153

Review URL: https://codereview.chromium.org/494653002

git-svn-id: svn://svn.chromium.org/blink/trunk@180656 bbb929c8-8fbe-4397-9dbb-9b2b20218538