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This reverts commit 5d01e87e.

Reason for revert: Caused regression https://crbug.com/1014385

Original change's description:
> Define guest view custom elements synchronously
> 
> Previously, we've had to wait for readystatechange before defining
> the guest view custom elements because
> 1) there were circular dependencies that would cause errors if we
> tried to perform the definition immediately, and
> 2) the Custom Elements V0 registration context was scoped to the
> document rather than the window, so if we defined the elements
> while the document was still on about:blank, the definition would
> be available on about:blank, but not for the extension.
> 
> Now that the circular dependencies are fixed and guest view is
> migrated to Custom Elements V1 (whose registration context is
> scoped to the window), it is no longer necessary to wait for
> readystatechange.
> 
> We now perform the definition immediately.
> 
> Bug: 810012
> Change-Id: Ib5e8ba6f4e302eaae56c5566765521d64b1a0e15
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1278287
> Commit-Queue: Kevin McNee <mcnee@chromium.org>
> Reviewed-by: James MacLean <wjmaclean@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#700425}

TBR=wjmaclean@chromium.org,mcnee@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 810012, 1014385
Change-Id: Ie43a88bc07036fc392079cc8f2593e28440de104
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863691Reviewed-by: Kevin McNee <mcnee@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706200}