Procs return the result by increasing recursion through result_for.

This CL eliminates that mechanism from the Processor interface and uses boolen
return values for sending tests to signal success or the failure to load the
test into the execution queue.

R=machenbach@chromium.org
CC=​​yangguo@chromium.org,sergiyb@chromium.org

Bug: v8:8174,v8:8731
Change-Id: I073a86ca84bcf88da11132b90013d4c8455bc61e
Reviewed-on: https://chromium-review.googlesource.com/c/1439239
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59201}