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Currently, empty CSSStyleSheets can only be constructed using a constructor method.
This CL adds Document.createEmptyCSSStyleSheet so that it can be tied to document in the future.
Tying it to document restricts the use of a CSSStyleSheet in different documents, which means CSSStyleSheets can only be used in the documents where they are constructed. This will reduce security risk.

Note:
The constructed CSSStyleSheet is not currently tied to the Document yet
Constructor method will be deleted in another CL
createEmptyCSSStyleSheet(CSSStyleSheetInit) produces an empty CSSStyleSheet, while createCSSStyleSheet(text, CSSStyleSheetInit) creates a CSSStyleSheet with text

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: I94ea6f795deaf0dee67fba5c2705c8749ac72da8
Reviewed-on: https://chromium-review.googlesource.com/1160422
Commit-Queue: Momoko Sumida <momon@google.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581169}