Use "unsigned" rather than "unsigned long" for C++ code interacting with WebIDL's "unsigned long".

(WebIDL's "unsigned long" is a 32-bit type)

BUG=267360

Review URL: https://codereview.chromium.org/722893002

git-svn-id: svn://svn.chromium.org/blink/trunk@185261 bbb929c8-8fbe-4397-9dbb-9b2b20218538