When using ninja to build without specifying explicit targets,
all existing targets in any BUILD.gn file are built/executed.

We now hide the snapshot targets behind the snapshot condition
to prevent them from being built and executed in nosnap builds.

CQ_INCLUDE_TRYBOTS=master.tryserver.v8:v8_linux_nosnap_rel

Bug: v8:7089
Change-Id: I4cd8ebadc377fd20b3887e9628990a75732ab74c
Reviewed-on: https://chromium-review.googlesource.com/778320Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49477}