Reland of "Remove EventSender from ImageLoader"

Reland of "Remove EventSender from ImageLoader"

Instead of using EventSender, this CL schedules ImageLoader's
load and error events using PostCancellableTask.
has_pending_load_event_ and has_pending_error_event_ booleans are
replaced with TaskHandle to the cancellable tasks and IsActive() calls.

This CL removes the dependencies between Document's load event and
<img> load events via EventSender.

This CL removes EventSender.h as ImageLoader was the last user of that.

Test changes:
stop-loading.html:
  testharness.js waits for the window load event before finishing.
  Previously, we call window.stop() in <img> load event, but window
  load event is fired, because the <img> load event is fired from
  Document::ImplicitClose() via EventSender. At that time window.stop()
  doesn't stop the window load event.
  After this CL, <img> load event is fired separately from
  Document::ImplicitClose(), and thus window.stop() stops the window
  load event.
  To work around this, this CL triggers window load event explicitly
  just to signal testharness.js to finish.

http/tests/loading/preload-image-sizes-2x.html and
http/tests/loading/preload-picture-sizes-2x.html:
  The tests uses srcset-helper.js that reloads the page with empty
  cache in the window load event.
  Because this CL changes the timing of the window load event, this
  causes some asynchronous tasks that cause new image loading to be
  scheduled before window load event and executed after that, causing
  some images left on the cache before reloading starts.
  This CL works around the issue by waiting a little bit for those
  tasks to be executed, and then clearing the cache and reloading the
  page.

BUG=624697, 720268
Review-Url: https://codereview.chromium.org/2863763003
Cr-Commit-Position: refs/heads/master@{#474086}
Committed: https://chromium.googlesource.com/chromium/src/+/da70e32d811fffe4e15728ba858dbdfa3180ed59

patch from issue 2863763003 at patchset 180001 (http://crrev.com/2863763003#ps180001)

[hiroshige, 2017-05-24 21:42:45]

The CQ bit was checked by hiroshige@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-24 21:43:50]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-25 01:01:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-05-25 01:01:18]

Dry run: This issue passed the CQ dry run.