Delete constructor for creating empty CSSStyleSheets

Currently, empty CSSStyleSheets can be constructed either with a constructor or with Document.createEmptyCSSStyleSheet.
This CL deletes the constructor so that they can only be produced by Document.createEmptyCSSStyleSheet.

Document.createEmptyCSSStyleSheet is considered to be more desirable, as CSSStyleSheets produced by Document.createEmptyCSSStyleSheet can be tied to documents in the future. This means that their use can be limited in the documents where they were produced, resulting in higher security.

Note:
The constructed CSSStyleSheet is not currently tied to the Document yet

Link to related comments in discussion:
https://github.com/WICG/construct-stylesheets/issues/23#issuecomment-379180786
https://github.com/WICG/construct-stylesheets/issues/15#issuecomment-391216056

Bug: 807560
Change-Id: I767e15e83e1f31eb278bc81233c8b579d0f511c7
Reviewed-on: https://chromium-review.googlesource.com/1164876
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Momoko Sumida <momon@google.com>
Cr-Commit-Position: refs/heads/master@{#581836}
8 files changed