Introduce CustomElementRegistry#get() method
This patch introduces |CustomElementRegistry#get()| as specified in [1].
To return associated constructor by |get()|, this patch introduces
|getConsturcotr(ScriptState*)| member function to |CustomElementDefinition| as
pure virtual function and implements it in |ScriptCustomElementDefinition|.
[1] https://html.spec.whatwg.org/multipage/scripting.html#dom-customelementsregistry-get
BUG=594918
TEST=LayoutTests/custom-elements/spec/custom-elements-registry/get.html
TEST=LayoutTests/webexposed/global-interface-listing.html
Review-Url: https://codereview.chromium.org/1994093002
Cr-Commit-Position: refs/heads/master@{#397351}
8 files changed
