Implement spec-compliant HTMLElement.prototype.isContentEditable

Web-exposed isContentEditable and our style-based editability are different
concepts.  For example, the former should be |true| even if an element has display:
none and users can't edit it.
So, we don't need to use the style-based editability to implement isContentEditable.

Specifications:
 https://html.spec.whatwg.org/multipage/interaction.html#dom-iscontenteditable
 https://w3c.github.io/editing/execCommand.html#editing-host
 https://w3c.github.io/editing/execCommand.html#editable

BUG=313082

Review-Url: https://codereview.chromium.org/2179273003
Cr-Commit-Position: refs/heads/master@{#408035}
4 files changed