Fixes the value of V8_VERSION_STRING to add the embedder string when the
patch level is zero.

BUG=v8:5740

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Idfe2020fe857865ff75a91ddb57f18ed23dfe3a7
Reviewed-on: https://chromium-review.googlesource.com/732992
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48834}