validation_pool: Split out throttled tree logic.

ApplyPoolIntoRepo contained the logic to decide what CLs to test if the
tree was logic. Move that logic out into it's own method.

This is prep work to allow intermediate steps that modify the manifest
to run after we know exactly what CLs are being considered, but before
any of them are applied into the code base.

BUG=chromium:549844
TEST=cbuildbot/run_tests

Change-Id: I0eb17459c956e4c1fe9e2995e9bd601f279aee72
Reviewed-on: https://chromium-review.googlesource.com/311212
Commit-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
3 files changed