In Tolkien’s The Lord of the Rings, the Stewards of Gondor also sat on a chair on the steps below the real throne, awaiting the return of the king. The prophecy in that case also included a magic sword, although Tolkien neglects to make any mention of a strawberry-shaped birthmark.
Other occurrences of the legend can be found in Robert Jordan’s The Wheel of Time epic fantasy series, in Raymond E. Feist’s Prince of the Blood, and in David Eddings’ Belgariad quintet.
This is undoubtedly one of those cases where everybody is drawing on a much older idea. Legends about kings, swords and birthmarks are of course legion, although I must admit that so far I haven’t been able to actually find an occurrence of the ‘chair below the real throne’ concept outside of contemporary fiction.