Also replaced kTrue/kFalse with a kBoolean type, and remove now-unused
IsTypeX() methods (leaving ones that are called frequently).

Bug: v8:6984
Change-Id: I0cbffc37efaa391981d8dce564051ce43257ed8a
Reviewed-on: https://chromium-review.googlesource.com/745023Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49053}