CSA::AllocateSeq{One,Two}ByteString used its own home-grown handling to
allocate very large strings. This CL refactors both methods to use
AllocationFlags::kAllowLargeObjectAllocation instead. Callers now need
to specify explicitly if large-object allocation is possible or not.

Bug: chromium:636391
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I0b7ffb0b083f4e977cea42c500f8f2ee1c60519f
Reviewed-on: https://chromium-review.googlesource.com/625738Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47504}