If an element A has a parent element, then A's assigned context menu is the assigned context menu of its parent element. Otherwise, A's assigned context menu is null.

Specification: https://html.spec.whatwg.org/multipage/forms.html#assigned-context-menu

"Each element has an assigned context menu, which can be null. If an element A has a contextmenu attribute, and there is an element with the ID given by A's contextmenu attribute's value in A's home subtree, and the first such element in tree order is a menu element whose type attribute is in the popup menu state, then A's assigned context menu is that element. Otherwise, if A has a parent element, then A's assigned context menu is the assigned context menu of its parent element. Otherwise, A's assigned context menu is null."

BUG=424494

Review URL: https://codereview.chromium.org/721603002

git-svn-id: svn://svn.chromium.org/blink/trunk@185198 bbb929c8-8fbe-4397-9dbb-9b2b20218538