Keep deps_os entries in dependencies, just with should_process set to False
for entries not applicable to target OS list.

This way gclient flatten has proper access to dependencies e.g. to evaluate
recursedeps referring to deps_os entries other than active OS.

Allow but ignore deps_os overriding a value with None, since that does not
fit the new model. There's no correctness harm in not checking out a repo.

Allow "overrides" setting given dependency to the same value. This seems
fairly common, especially for mac/ios and unix/android, even in chromium/src.

Bug: 570091
Change-Id: I2037a1ecc5fd2da6b5f73061548b81fc79ba2e72
Reviewed-on: https://chromium-review.googlesource.com/541280Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>