Suppose, you have 3 chained changes, each using submit strategy=Rebase Always:
 <common base on master> -> A > B -> C

if all changes are approved and CQ is triggered on C, 
then what should happen?

Currently:
  CQ will try  (A+B+C) on top of current master HEAD,
  if try jobs pass, land A, B, C.
  => there is no guarantee that change A or A+B didn't result in compile failures.

What should happen? Either:

1. CQ rejects a full run on C immediately if B hasn't landed === Rietveld behavior.

2. CQ triggers all tryjobs, but if all tryjobs pass, fail with "must submit B first" if B hasn't yet landed.

3. CQ magically triggers CQ runs on B and waits for it to complete before issuing "submit" on C. CQ run on B will do the same with A, the end result being roughly this timeline:

  user triggers CQ on C
  CQ C: triggers CQ on B
  CQ C: triggers tryjobs on A+B+C
  CQ B: triggers CQ on A
  CQ B: triggers tryjobs on A+B
  CQ A: triggers tryjobs on A
  
  CQ B: notices that all tryjobs A+B pass, but must wait for CQ A to finish running
  CQ A: notices that all tryjobs A pass, submits A
  CQ B: notices CQ A is complete, submits B
  CQ C: notices that A+B+C pass and that CQ B finished, submits C.