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If a frame has guest view custom elements defined, then any iframes also
need the definition or else a webview in an iframe will not be
initialized. If the iframe contains a script element, a script context
will be created which allows for the definition of the custom element in
the iframe. If there are no script elements, then no script context is
created, so the custom element definition is not available.

We make the ForceMainWorldInitialization setting available so that
extension code can force the creation of a script context for any
subframe of a frame which has access to guest view custom element
definitions.

Bug: 788914
Change-Id: Ibdd7c321b6e67dbe913f3deb2307406102d7b44a
Reviewed-on: https://chromium-review.googlesource.com/828045
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531744}