This adds a new gn arg v8_custom_deps to add a dependency that's dynamically
added under v8/custom_deps. E.g. for depending on a target in
v8/custom_deps/test_root/BUILD.gn set
v8_custom_deps = "custom_deps/test_root:target_name".

Bug: chromium:846711
Change-Id: If03d48dba07cb36dc1b981602e5dc267deec086b
Reviewed-on: https://chromium-review.googlesource.com/1095218Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53640}