IDL: Support union type arrays and sequences for method arguments

toImplArray() template can convert arrays (or sequences) of T to
Vector<T> when T has NativeValueTraits. Generate them for union type
containers so that we can use (A or B)[] (or sequence<(A or B)>)
in method arguments.

Note that we can't use union type arrays and sequences for
return values yet. Follow-up CLs will support return values.

BUG=240176

Review URL: https://codereview.chromium.org/700733003

git-svn-id: svn://svn.chromium.org/blink/trunk@185096 bbb929c8-8fbe-4397-9dbb-9b2b20218538