The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1c5d3f11a80d88d65f5fae8398ed97a7b5ea1508

commit 1c5d3f11a80d88d65f5fae8398ed97a7b5ea1508
Author: hiroshige <hiroshige@chromium.org>
Date: Mon May 15 22:28:51 2017

Partial reland of "Add CHECK()s about ImageLoader::has_pending_load_event_"

Original CL:
https://codereview.chromium.org/2859383002/#ps220001

This CL relands the CHECK()s except for the failing one in
DispatchPendingErrorEvent().

 Issue 722500  tracks the issue around DispatchPendingErrorEvent() and relanding
of the remaining CHECK().

Original CL description:
> As preparation for [1], this CL checks some assumptions to hold that
> are helpful for proving the equivalences in [1].
>
> [1] https://codereview.chromium.org/2859093003/
>
> BUG= 624697 ,  719759 
>
> Review-Url: https://codereview.chromium.org/2859383002
> Cr-Commit-Position: refs/heads/master@{#471464}
> Committed: https://chromium.googlesource.com/chromium/src/+/1b070387595668ec438ffec65185e61017300e3c

BUG= 624697 ,  719759 ,  722500 

Review-Url: https://codereview.chromium.org/2884773002
Cr-Commit-Position: refs/heads/master@{#471925}

[modify] https://crrev.com/1c5d3f11a80d88d65f5fae8398ed97a7b5ea1508/third_party/WebKit/Source/core/loader/ImageLoader.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/84fb9e0a3e168b81e538a5ff8ce79e3690396a10

commit 84fb9e0a3e168b81e538a5ff8ce79e3690396a10
Author: hiroshige <hiroshige@chromium.org>
Date: Fri May 19 16:53:20 2017

Make ImageLoader have at most one pending error event on EventSender

Relanding the CHECK() that asserts
ImageLoader::DispatchPendingErrorEvent() should always called when
has_pending_error_event_ is true.

Previously, we sometimes schedule multiple error events on EventSender:
1. DispatchErrorEvent() is called.
2. DispatchErrorEvent() is called.
3. DispatchPendingErrorEvent() is called (corresponding to Step 1)
   and actual processing is done.
4. DispatchPendingErrorEvent() is called (corresponding to Step 2)
   but ignored because has_pending_error_event_ is already false.

This CL makes DispatchErrorEvent() cancel previous scheduled error
events if any, ensures there can be at most scheduled error event on
EventSender, and makes the assertion hold.

The scenario above will be processed as follows, and thus this CL
shouldn't change any observable behavior.
1. DispatchErrorEvent() is called.
2. DispatchErrorEvent() is called.
   The error event scheduled by Step 1 is cancelled.
3. DispatchPendingErrorEvent() is called (corresponding to Step 2)
   and actual processing is done.

[1] https://codereview.chromium.org/2859383002/
[2] https://codereview.chromium.org/2884773002/

BUG= 624697 ,  719759 ,  722500 

Review-Url: https://codereview.chromium.org/2893993002
Cr-Commit-Position: refs/heads/master@{#473214}

[add] https://crrev.com/84fb9e0a3e168b81e538a5ff8ce79e3690396a10/third_party/WebKit/LayoutTests/images/multiple-inflight-error-event-crash.html
[modify] https://crrev.com/84fb9e0a3e168b81e538a5ff8ce79e3690396a10/third_party/WebKit/Source/core/loader/ImageLoader.cpp

The CL at Comment #4 was landed on 60.0.3105.0 and no related crashes are observed.