Replace [TreatReturnedNullStringAs=Null] with nullable return type

There are no changes to the generated code.

One instance of TreatReturnedNullStringAs=Null remains for
Node.textContent, where removing it does change the generated code
because TreatUndefinedAs=NullString is also involved.

BUG=460722

Review URL: https://codereview.chromium.org/950763002

git-svn-id: svn://svn.chromium.org/blink/trunk@190643 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed
tree: 1d7a1811f14b459bf882c5702bafd3d715c56fee
  1. third_party/