Remove getMediaSourceIdForWebContents()

Editor assessment

The subject states the removal: the getMediaSourceIdForWebContents function deleted. The feat type is a questionable fit for an API removal, though reverts often straddle; the body clarifies little beyond a standard revert notice naming the undone commit, with no word on why the addition was withdrawn or what replaces the identifier lookup. No scope is present. The PR reference is well-formed. Understandable as an undo, but the absent rationale — never released, or superseded — keeps the message at the acceptable tier.