Only overrides v8_current_cpu in mac toolchain when declared

Sometimes we expect to build mac target without v8, but there is a
warning that v8_current_cpu is set as a build argument, but not
appeared in declare_args(). If the declaration file v8_target_cpu.gni
is not imported, we should not override it.

Change-Id: I04ef533a565fb9272aa1117ccea1bdee022f5c9a
Reviewed-on: https://chromium-review.googlesource.com/c/1369645
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615530}
2 files changed