On second thought, the proposed short-fix has the potential to run into an issue, where keeping 2 webcontents in memory (even for a short time) may raise our chances of OOM issues.  In effort to delay this until actual experimentation regarding 2+ background loaders, we have alternative fix:

- Offliner::Cancel() will be changed to async (takes a callback)
- in PrerenderOffliner::Cancel(), the callback will be called immediately
- in BackgroundLoaderOffliner::Cancel(), the callback will be called immediately if we're not in a Save() operation, or after the Save()
- in RequestCoordinator, when Cancel() is called, we will do whatever comes post-cancel inside the callback

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9f04f39d54f599a346c59670f91923f4af05e416

commit 9f04f39d54f599a346c59670f91923f4af05e416
Author: chili <chili@chromium.org>
Date: Sat Mar 04 01:23:56 2017

[Offline Pages] Turn Offliner::Cancel into an async operation to resolve conflicting assumptions
between the Offliner and RequestCoordinator.

Upon Offliner::Cancel, RequestCoordinator assumes that the offliner is free, but
offliner may be waiting until save operation to complete. This discrepancy causes
RequestCoordinator to bombard the offliner with all the pending requests, potentially
blocking the main thread for the save operation callback to be called and causing
all pending requests to be labeled as EXPIRED

BUG= 694864 

Review-Url: https://codereview.chromium.org/2715433006
Cr-Commit-Position: refs/heads/master@{#454740}

[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/chrome/browser/android/offline_pages/background_loader_offliner.cc
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/chrome/browser/android/offline_pages/background_loader_offliner.h
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/chrome/browser/android/offline_pages/background_loader_offliner_unittest.cc
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/chrome/browser/android/offline_pages/prerendering_offliner.cc
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/chrome/browser/android/offline_pages/prerendering_offliner.h
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/components/offline_pages/core/background/offliner.h
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/components/offline_pages/core/background/offliner_stub.cc
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/components/offline_pages/core/background/offliner_stub.h
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/components/offline_pages/core/background/request_coordinator.cc
[modify] https://crrev.com/9f04f39d54f599a346c59670f91923f4af05e416/components/offline_pages/core/background/request_coordinator.h

Your change meets the bar and is auto-approved for M58. Please go ahead and merge the CL to branch 3029 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), bhthompson@(cros), govind@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dedea73d3b658d752873cadbc162c84f88bcd878

commit dedea73d3b658d752873cadbc162c84f88bcd878
Author: Dmitry Titov <dimich@chromium.org>
Date: Fri Mar 10 01:24:15 2017

[Offline Pages] Turn Offliner::Cancel into an async operation to resolve conflicting assumptions between the Offliner and RequestCoordinator.

Upon Offliner::Cancel, RequestCoordinator assumes that the offliner is free, but
offliner may be waiting until save operation to complete. This discrepancy causes
RequestCoordinator to bombard the offliner with all the pending requests, potentially
blocking the main thread for the save operation callback to be called and causing
all pending requests to be labeled as EXPIRED

BUG= 694864 

Review-Url: https://codereview.chromium.org/2715433006
Cr-Commit-Position: refs/heads/master@{#454740}
(cherry picked from commit 9f04f39d54f599a346c59670f91923f4af05e416)

Review-Url: https://codereview.chromium.org/2744553005 .
Cr-Commit-Position: refs/branch-heads/3029@{#105}
Cr-Branched-From: 939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}

[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/chrome/browser/android/offline_pages/background_loader_offliner.cc
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/chrome/browser/android/offline_pages/background_loader_offliner.h
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/chrome/browser/android/offline_pages/background_loader_offliner_unittest.cc
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/chrome/browser/android/offline_pages/prerendering_offliner.cc
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/chrome/browser/android/offline_pages/prerendering_offliner.h
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/chrome/browser/android/offline_pages/prerendering_offliner_unittest.cc
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/components/offline_pages/core/background/offliner.h
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/components/offline_pages/core/background/offliner_stub.cc
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/components/offline_pages/core/background/offliner_stub.h
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/components/offline_pages/core/background/request_coordinator.cc
[modify] https://crrev.com/dedea73d3b658d752873cadbc162c84f88bcd878/components/offline_pages/core/background/request_coordinator.h