Previously, Date.toString() and friends were completely
implementation-defined. However, they actually seemed to match
each other's behavior with the exception of how years less than
1000 are formatted. The rough consensus among browsers seemed
to be %04d, so this was standardized at TC39 [1]. V8 previously
used %4d (it was the only one to do so); this patch adopts
the new standard.

[1] https://github.com/tc39/ecma262/commit/5d4acf3377e1b0ad4a4660c57108db1b28ef20ff

Bug: v8:6076
Change-Id: I8c795a4e1b71187ad7c24a1aee8d7d66719a2586
Reviewed-on: https://chromium-review.googlesource.com/536733
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46037}