Allow executeTool() to execute tools registered by opaque-origin documents #349

Right now, executeTool() rejects with "NotSupportedError" when the tool's origin is opaque. Documents with an opaque origin can register tools and see them via getTools(), but can never execute them in-page. This is because the RegisteredTool dictionary's origin member isn't really an [=origin=] type, but is rather a string serialization of an origin. This loses the identity of opaque origins (holding only "null"), such that by the time an tool from an opaque origin reached executeTool() as "null", we couldn't compare it against the target document, and had to always reject.

Since #348, a Tool stores the tool owner's actual origin internally, even though Tool#origin is still exposed to the platform as a stringified serialization. This means we can finally perform same origin comparison against a Tool and the execution's target window.