Fix customized built-in element constructor behavior

[chromium-wpt-export-bot]

This CL implements two changes:

1. It fixes the implementation to better match the spec for the
   "create an element for the token" [1] algorithm. Prior to this CL,
   step 7 of that algorithm was skipping directly to step 6 of the
   "create an element" [2] algorithm, skipping over step 5 for
   customized built-in elements. This is now fixed. This case is
   illustrated by the issue and example at [3] and [4]. This becomes
   the first test in customized-built-in-constructor-exceptions.html.

2. It updates the comments to match the new behavior discussed in [3]
   and the [5] spec PR, which changes the return value in the case
   that a customized built-in element constructor throws an exception.
   With the change above, that is actually already the behavior. So
   this is just a comment change. Two new tests are added to
   customized-built-in-constructor-exceptions.html.

[1] https://html.spec.whatwg.org/multipage/parsing.html#create-an-element-for-the-token
[2] https://dom.spec.whatwg.org/#concept-create-element
[3] whatwg/html#5084
[4] https://crbug.com/1024866
[5] whatwg/dom#797

Bug: 1071059, 1024866
Change-Id: I814c81991eb5e83501304bcb3d2da476743aef52
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152986
Commit-Queue: Mason Freed <[email protected]>
Auto-Submit: Mason Freed <[email protected]>
Reviewed-by: Kent Tamura <[email protected]>
Cr-Commit-Position: refs/heads/master@{#760705}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.