jumbo.gni will check whether use_jumbo_build is set or not and
if it's not set it will just check that |jumbo_excluded_sources|
lists do not include non-existing files. That means that there
is little/nothing to lose by always routing v8_source_set that
way, and it adds a bit in build file simplicity and local
checking of |jumbo_excluded_sources|.

Change-Id: I19ed91d304f4a2edbf10661d20e5f5d3180b3a35
Reviewed-on: https://chromium-review.googlesource.com/c/1430832
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59169}