Change "bool createdByParser" of createElement() to enum

The "create an element"[1] defines "synchronous custom elements flag"
that is set to false if:
1. To clone a node[2].
2. The parser was originally created for the HTML fragment parsing
   algorithm[3].

This patch changes "bool createdByParser" of createElement() to an enum
so that callers can pass this flag to "create an element"[1].

Using the "synchronous custom elements flag" is not implemented yet,
and thus no behavior changes.

[1] https://dom.spec.whatwg.org/#concept-create-element
[2] https://dom.spec.whatwg.org/#concept-node-clone
[3] https://html.spec.whatwg.org/#create-an-element-for-the-token

BUG=594918

Review-Url: https://codereview.chromium.org/2002063003
Cr-Commit-Position: refs/heads/master@{#396125}
15 files changed