bindings: Renames internalPointer to scriptWrappableBase.

This is just renaming.

BUG=235436

Review URL: https://codereview.chromium.org/663063003

git-svn-id: svn://svn.chromium.org/blink/trunk@184067 bbb929c8-8fbe-4397-9dbb-9b2b20218538