Just as we deem the legislative intent manifest to tax the stockholder with respect to such accumulations only if and when, and to the extent that, his interest in them comes to fruition as income, that is, in dividends declared, so we can perceive no constitutional obstacle that stands in the way of carrying out this intent when dividends are declared out of a pre-existing surplus... "Congress was at liberty under the amendment to tax as income, without apportionment, everything that became income, in the ordinary sense of the word, after the adoption of the amendment, including dividends received in the ordinary course by a stockholder from a corporation", even though they were extraordinary in amount and might appear upon analysis to be a mere realization in possession of an inchoate and contingent interest that the stockholder had in a surplus of corporate assets previously existing. added