This is a reland of https://chromium-review.googlesource.com/c/541280/
with a fix for https://bugs.chromium.org/p/chromium/issues/detail?id=735418
(patchset 1 is original patch, patchset 2 has the fix).

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, 735418
Change-Id: I6eba0e4be202212eb86cb959c18f2b2f0c1452b9
Reviewed-on: https://chromium-review.googlesource.com/543076
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>