This CL implements custom elements. The design is as follows:
1) Authors subclass Element and call registerElement.
2) When we create C++ elements for custom elements, we call the author's
constructor synchronously.
3) The attach/detach/attributeChanged callbacks are called either:
a) when exiting the current custom element callback scoped (e.g., before
returning from appendChild), or
b) when draining the microtask queue.
The implementation in this CL is a bit fragile because we don't detect name
registration conflicts and we let you create custom elements with the same name
as built-in elements. Also, not every part of the engine is prepared to execute
script synchronously below createElement. We'll need to iron out these issues
over time, but this CL is a start.
R=ojan@chromium.org
Review URL: https://codereview.chromium.org/943013002
