[chromium] Add WebKit API to access <datalist> options

[chromium] Add WebKit API to access <datalist> options
https://bugs.webkit.org/show_bug.cgi?id=83762

Introduce WebInputElement::dataListOptions so we can implement a UI for datalist.

Reviewed by Kent Tamura.

* public/WebInputElement.h:
(WebKit):
(WebInputElement):
* src/WebInputElement.cpp:
(WebKit::WebInputElement::dataListOptions): Added. Returns a WebNodeCollection containing WebOptionElements.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114108 268f45cc-cd09-0410-ab3c-d52691b4dbfc