[webcomponents] What was the "element is being created" flag for? (#458)

@dglazkov, need help again.

Previous revisions of the spec, when createdCallback existed, had this text under createdCallback:

> For the duration of this callback invocation, the element is being created flag MUST be set to true. In all other cases, the flag MUST be set to false.

> If the created callback exists for an element, all other callbacks MUST NOT be enqueued until after this created callback's invocation had started.

Furthermore, both previous revisions and the current spec have a clause:

> If element's element is being created flag is false, add element to the current element queue.

inside the enqueuing of custom element callbacks. That is, if the element is being created:

- The relevant reaction _will_ be added to element's custom element reaction queue, but
- element _will not_ be added to the current element queue (i.e. to the queue on the top of the stack-of-queues)

I am trying to figure out what the intent was here, and whether we need to reintroduce setting "element is being created" before/after calling the constructor in the current design, or whether we should remove the "element is being created" flag and the resulting fork in the enqueuing logic.

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