IDL compiler: Nullable callback interface arguments

One use:
Source/core/dom/DataTransferItem.idl:
[CallWith=ExecutionContext] void getAsString(StringCallback? callback);

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/127773002

git-svn-id: svn://svn.chromium.org/blink/trunk@164732 bbb929c8-8fbe-4397-9dbb-9b2b20218538