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When the newly created WebContents through chrome.windows.create doesn't
wish to preserve opener relationship, do not specify
NavigateParams.source_site_instance as it as it is only required and
useful when there is a need to keep two WebContentses in the same
BrowsingInstance or SiteInstance. Further, this makes WindowsCreateFunction
only use SiteInstances when necessary.

Add tests to cover the test cases that also verify that the newly created
WebContents has correct origin or not.

Bug: 986348
Change-Id: I679354bb77c317c4ecc4aeabfec8799bca69dd25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712441
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681913}