(triage) needs state machine documentation or design doc

If we write a design doc, this bug might be a good candidate for a fixit.

Design doc here, this bug is ready to go: https://docs.google.com/a/google.com/document/d/12Kltx2Cd_gsIOPbGNmlhIosI2EiaKmVfT_YZhihhI_Y

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7be44a4d4b5fbbd8787a57a4ffb705dae78bf7e1

commit 7be44a4d4b5fbbd8787a57a4ffb705dae78bf7e1
Author: romax <romax@chromium.org>
Date: Thu Mar 30 17:58:17 2017

[Offline Pages] Improve RequestCoordinator state tracking.

Replace the several boolean values used for state tracking with single
state variable.

BUG= 644519 

Review-Url: https://codereview.chromium.org/2775223006
Cr-Commit-Position: refs/heads/master@{#460820}

[modify] https://crrev.com/7be44a4d4b5fbbd8787a57a4ffb705dae78bf7e1/components/offline_pages/core/background/request_coordinator.cc
[modify] https://crrev.com/7be44a4d4b5fbbd8787a57a4ffb705dae78bf7e1/components/offline_pages/core/background/request_coordinator.h
[modify] https://crrev.com/7be44a4d4b5fbbd8787a57a4ffb705dae78bf7e1/components/offline_pages/core/background/request_coordinator_unittest.cc