IsString should not count FIRST_NON_STRING_TYPE as a string type.

BUG=

Change-Id: I093945763c3d2b56de5b4b5745ba207fe2d8b5d1
Reviewed-on: https://chromium-review.googlesource.com/453899Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43783}