IDL: Remove the CompareHow special case

It's a special type used only for Range.compareBoundaryPoints()'s |how|
argument, and all the bindings layer actually does is cast a 32-bit int to
the Range::CompareHow C++ enumeration, without any checks. The spec's IDL
has the argument's type as |unsigned short|.

This doesn't change any behavior (the value checks were always in the C++
code, and still are), aligns our IDL files with the spec, and, most
significantly, removes the special case from the code generator.

BUG=413257

Review URL: https://codereview.chromium.org/592293003

git-svn-id: svn://svn.chromium.org/blink/trunk@182498 bbb929c8-8fbe-4397-9dbb-9b2b20218538