In https://chromium-review.googlesource.com/c/chromium/src/+/970424 we turned on the FindBadConstruct clang plugin's no-realpath flag, which worked in chromium builds, but broke standalone V8 builds:

> Ahh, I see the problem my change caused: the "/v8/" pattern in
> banned_directories_ works when building the chromium tree because
> paths to v8 sources relative to the out dir look like "../../v8/foo",
> whereas in a standalone v8 build the equivalent path would be
> "../../foo".  

> Things just happen to work in the standalone v8 build because the
> checkout location is a directory named "v8" on the bots, so the
> absolute path contains this and then all sources are considered
> "banned".  I don't think the style checker plugin should make
> assumptions about the location of the root checkout dir, but I
> need to think some more about how this should be implemented.

So prevent this from blocking V8 rolls, the plugin was disabled in
standalone v8 builds:
https://chromium-review.googlesource.com/998153

To avoid more trouble, let's re-enable the plugin in standalone v8
builds in such a way that v8 code is not misclassified as "chromium"
code, so we still run the tests that chromium builds run on "third party"
code.