[Find Element(s) From Shadow Root: Tag name strategy] Impossible to call `getElementsByTagName()` for ShadowRoot

[yezhizhen]

Following two are common steps for "Find Element(s) From Shadow Root"

1. Let start node be the result of trying to get a known shadow root with session and URL variables["shadow id"].
2. Let result be the value of trying to Find with session, start node, location strategy, and selector.

However, if the element location strategy is Tag name, this is impossible, as getElementsByTagName is only a method for Document or Element.