Add kInternalizedNonEmptySeqString and make kInternalizedString = 
kEmptyString | kInternalizedNonEmptySeqString.

BUG=chromium:731495

Change-Id: I60a6f37242423407ad97188b3f4bd48ae6989ad2
Reviewed-on: https://chromium-review.googlesource.com/528275Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45821}