IDL clean-up: Rename is_nullable_simple => is_implicit_nullable

... and introduce is_explicit_nullable that is mutually exclusive (for
nullable types.)

It turns out that the only actual use of is_implicit_nullable right now
is in the definition of is_explicit_nullable, but having both is fine,
for symmetry.

Review URL: https://codereview.chromium.org/384773004

git-svn-id: svn://svn.chromium.org/blink/trunk@177912 bbb929c8-8fbe-4397-9dbb-9b2b20218538