Cleanup [CustomConstructor] usage

Cleanup, no changes to generated code.

The main point of this CL is that it removes use of
[Constructor, CustomConstructor] in XMLHttpRequest.idl.
The [Constructor] is a nop (if you have [CustomConstructor], we don't
generate constructors), so this CL removes it.
(Noticed this when porting [CustomConstructor].)

Also minor lint fixes.

Review URL: https://codereview.chromium.org/111683006

git-svn-id: svn://svn.chromium.org/blink/trunk@163949 bbb929c8-8fbe-4397-9dbb-9b2b20218538