Add enum values to the "no cache reason" API which reflect new types of
no-cache reason we will want to distinguish.

Also, renames one of the enum values (BecauseExtension ->
BecauseV8Extension) because it was confusing. It's a V8-only type of no
cache reason, so it shouldn't affect embedders.

Bug: chromium:769203
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I41d4ecfb35b2e91b71562b4f23b15d20f16a943c
Reviewed-on: https://chromium-review.googlesource.com/769010Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49367}