Bindings: IDL validation that overloads consistently return Promises

Operation overloads that inconsistently return Promises may violate
Web IDL. Blink doesn't have any, and doesn't consistently implement
arity checking for overloads that return Promises, and the Web IDL
issue is not completely resolved [1]... so let's avoid introducing any
new cases.

[1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=26517

BUG=359386
R=yhirano@chromium.org,haraken@chromium.org

Review URL: https://codereview.chromium.org/523573003

git-svn-id: svn://svn.chromium.org/blink/trunk@181244 bbb929c8-8fbe-4397-9dbb-9b2b20218538