Reland Simplify CanvasAsyncBlobCreator by removing ContextObserver and tests

Reland Simplify CanvasAsyncBlobCreator by removing ContextObserver and tests

After we confirm to use idle-tasks implementation in toBlob, the cancellation
mechanism for async thread implementation is no longer useful. First, compositor
thread is definitely alive when users open Chrome browser; so the alternative
code path for PNG image encoding when compositor thread is disabled would be
unreachable in real-world situations; as a result, PNG image encoding happens
in idle tasks only. Second, JPEG and WEBP image formats do not have progressive
encoding at this moment and thus cancellation mechanism is not useful to them
either; once their progressive encoding implementations are completed, they
would be moved over to idle-tasks implementation too.

This patch removes ContextObserver and CanvasAsyncBlobCreatorTest; it also moves
all layout tests that use toBlob on PNG image formats to virtual/threaded as
idle tasks require compositor thread to be alive. There is no need to perform
tests on toBlob (png) in scenarios when compositor thread is not available.

BUG=581574
Committed: https://crrev.com/f6ff8ac1cd9aa17d32676efa883e520365405f98
Cr-Commit-Position: refs/heads/master@{#373567}

[xlai (Olivia), 2016-02-04 01:00:57]

Patchset #2 (id:20001) has been deleted

[xlai (Olivia), 2016-02-04 01:45:53]

xlai@chromium.org changed reviewers:
+ junov@chromium.org

[xlai (Olivia), 2016-02-04 01:45:54]

junov@: I fixed the leak problem which is the reason of the revert
(https://codereview.chromium.org/1660993002/). 

Basically, canvas-toBlob-invalid.html has 2 expected toBlob calls that are NOT
throwing exceptions; as a result, 2 async callbacks are still alive when the
layout test finishes, which leads to the leaks. We should make the layout test
finishes only after these async callbacks are executed.

Previously, canvas-toBlob-invalid.html has never been run in virtual/threaded
mode and thus not on idle periods on main thread; such "leaks" on async threads
are not captured and the test was always green (note that the test is green
before and after context observer is implemented, so it has nothing to do with
the contextObserver).

[Justin Novosad, 2016-02-04 16:41:57]

lgtm

[xlai (Olivia), 2016-02-04 16:57:42]

The CQ bit was checked by xlai@chromium.org

[commit-bot: I haz the power, 2016-02-04 16:58:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1669713002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1669713002/40001

[commit-bot: I haz the power, 2016-02-04 18:19:04]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2016-02-04 18:19:59]

Description was changed from

==========
Reland Simplify CanvasAsyncBlobCreator by removing ContextObserver and tests

After we confirm to use idle-tasks implementation in toBlob, the cancellation
mechanism for async thread implementation is no longer useful. First, compositor
thread is definitely alive when users open Chrome browser; so the alternative
code path for PNG image encoding when compositor thread is disabled would be
unreachable in real-world situations; as a result, PNG image encoding happens
in idle tasks only. Second, JPEG and WEBP image formats do not have progressive
encoding at this moment and thus cancellation mechanism is not useful to them
either; once their progressive encoding implementations are completed, they
would be moved over to idle-tasks implementation too.

This patch removes ContextObserver and CanvasAsyncBlobCreatorTest; it also moves
all layout tests that use toBlob on PNG image formats to virtual/threaded as
idle tasks require compositor thread to be alive. There is no need to perform
tests on toBlob (png) in scenarios when compositor thread is not available.

BUG=581574
==========

to

==========
Reland Simplify CanvasAsyncBlobCreator by removing ContextObserver and tests

After we confirm to use idle-tasks implementation in toBlob, the cancellation
mechanism for async thread implementation is no longer useful. First, compositor
thread is definitely alive when users open Chrome browser; so the alternative
code path for PNG image encoding when compositor thread is disabled would be
unreachable in real-world situations; as a result, PNG image encoding happens
in idle tasks only. Second, JPEG and WEBP image formats do not have progressive
encoding at this moment and thus cancellation mechanism is not useful to them
either; once their progressive encoding implementations are completed, they
would be moved over to idle-tasks implementation too.

This patch removes ContextObserver and CanvasAsyncBlobCreatorTest; it also moves
all layout tests that use toBlob on PNG image formats to virtual/threaded as
idle tasks require compositor thread to be alive. There is no need to perform
tests on toBlob (png) in scenarios when compositor thread is not available.

BUG=581574

Committed: https://crrev.com/f6ff8ac1cd9aa17d32676efa883e520365405f98
Cr-Commit-Position: refs/heads/master@{#373567}
==========

[commit-bot: I haz the power, 2016-02-04 18:20:00]

Patchset 2 (id:??) landed as
https://crrev.com/f6ff8ac1cd9aa17d32676efa883e520365405f98
Cr-Commit-Position: refs/heads/master@{#373567}