Only use sourceOrigin for all about:-schemed Documents

[domfarolino]

Before this PR, determine the origin took in two origins: sourceOrigin (returned for about:blank URLs) and containerOrigin (returned for about:srcdoc). But I don't think there's a reason for these two be different, so this CL combines them.

When navigating a srcdoc iframe via the srcdoc attribute, the parent is always used as the sourceDocument passed into the #navigate algorithm, which means that the container origin used here and here for srcdoc iframe navigations, is the initiator/source origin. That, coupled with the fact that there's no instance where we pass in sourceOrigin and containerOrigin together (for the algorithm to decide between), means they are exclusive, but both used for about:-schemed documents, and generally feel redundant IMO.

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/browsing-the-web.html ( diff )
/document-sequences.html ( diff )

[domfarolino]

@domenic LMK what you think about this. I personally kind of get confused with the two exclusive origin parameters for this algorithm, even though they basically represent the same thing, since the "initiator"/"source" for about:srcdoc navigations is always the parent, and we have no intention of changing that.

[domenic]

Great idea. I wonder how long this kind of simplification has been possible.

See also #8739 for the future.