Changes to element creation during parsing per F2F

[domenic]

During parsing, the moment the open tag is seen:

1. Increment the ignore destructive writes counter
2. Drop out of the parser, like you do for script elements
3. Queue the constructor.
4. Queue the attributeChangedCallbacks, in order, as you see new attributes.
5. Run the custom element callbacks queue, synchronously
6. If the return value of the constructor is not of the correct registered name, just create a HTMLUnknownElement instead and stop processing the queue
7. If any of the callbacks run throw, just create a HTMLUnknownElement instead and stop processing the queue
8. Decrement the counter
9. Resume parsing

Still TBD: does innerHTML do step "run the custom element callbacks queue, synchronously" or not? At the meeting we determined innerHTML uses nanotasks, like all other IDL APIs

[domenic]

This should now be implemented:

• https://w3c.github.io/webcomponents/spec/custom/#parsing
• https://w3c.github.io/webcomponents/spec/custom/#html-parsing-xhtml-documents

There are a couple rough edges:

• Step 9.1. is currently "Invoke custom element actions (for which queue!?! just for element maybe!?)" reflecting our/my lack of clarity about custom element callbacks and how they work or should work. That will be resolved as part of Integrate callback invocation with IDL #186.
• There's still ongoing discussion about the auto-upgrade flag; that's Custom elements disconnected from a document should not be upgraded #419. For now I've left it as fragment parsing does not auto-upgrade, but document parsing does. I expect that will evolve as Custom elements disconnected from a document should not be upgraded #419 does.

Review appreciated!