GitLab

Specs: https://html.spec.whatwg.org/#html-element-constructors

Replaced V8HTMLElementCustom with V8HTMLConstructor

Add new extended attribute [HTMLConstructor] to bindings and IDLExtendedAttributes.txt
  Added description to IDLExtendedAttributes.md and re-organized descriptions
  Added checks for conflicting extended attributes with [HTMLConstructor] in v8_interface.py
  Edited jinja templates for [HTMLConstructor] interface

Replaced [CustomConstructor] with [HTMLConstructor] in HTMLElement.idl

BUG=648828

Review-Url: https://codereview.chromium.org/2385073002
Cr-Commit-Position: refs/heads/master@{#423068}