Implement WTF::WeakPtr in terms of base::WeakPtr

Implement WTF::WeakPtr in terms of base::WeakPtr

BUG=645774
Committed: https://crrev.com/24430ece16aa10aa57644daeddafe25032fd359c
Cr-Commit-Position: refs/heads/master@{#418441}

[dcheng, 2016-09-10 18:58:30]

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

[commit-bot: I haz the power, 2016-09-10 18:58:38]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[dcheng, 2016-09-10 18:59:28]

Patchset #2 (id:20001) has been deleted

[commit-bot: I haz the power, 2016-09-10 19:00:08]

Dry run: Try jobs failed on following builders:
  ios-device on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device/builds...)
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)
  mac_chromium_rel_ng on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[dcheng, 2016-09-10 19:02:09]

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

[commit-bot: I haz the power, 2016-09-10 19:02:22]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[dcheng, 2016-09-10 19:04:11]

dcheng@chromium.org changed reviewers:
+ esprehn@chromium.org

[dcheng, 2016-09-10 19:04:11]

I'd like to reland this sooner rather than later; WeakPtr usage is going to
increase, so the more we end up depending on divergent behavior, the more
painful it's going to be to merge them.

For now, I've just cheated and made MainThreadTaskRunner preemptively create a
WeakPtr to avoid the data race when posting tasks.

In followup patches, I'll investigate the impact of:
- Making this thread safe by not lazily creating the internal flag in GetWeakPtr
- Reducing the memory impact of base::WeakPtr by storing the pointer inside the
shared flag

[commit-bot: I haz the power, 2016-09-10 20:33:48]

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

[commit-bot: I haz the power, 2016-09-10 20:33:49]

Dry run: This issue passed the CQ dry run.

[dcheng, 2016-09-12 21:07:31]

I built tsan locally and tested to verify that it all works.

Re: GetWeakPtr() / createWeakPtr() thread-safety. In theory, there's no reason
why this shouldn't be safe if we never use InvalidateWeakPtrs() / revokeAll().
But as mentioned in the bug, that's a discussion for another CL (if we want to
pursue it)

[esprehn, 2016-09-13 19:44:22]

lgtm, are you sure it's okay to share a weakptr like that? Why would we want to
make lots of them like before?

https://codereview.chromium.org/2329243002/diff/40001/third_party/WebKit/Sour...
File third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp (right):

https://codereview.chromium.org/2329243002/diff/40001/third_party/WebKit/Sour...
third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp:56: m_weakPtr,
This means we're sharing a weakptr now instead of making a new one per task?

[dcheng, 2016-09-13 19:51:41]

https://codereview.chromium.org/2329243002/diff/40001/third_party/WebKit/Sour...
File third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp (right):

https://codereview.chromium.org/2329243002/diff/40001/third_party/WebKit/Sour...
third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp:56: m_weakPtr,
On 2016/09/13 19:44:21, esprehn wrote:
> This means we're sharing a weakptr now instead of making a new one per task?

It's copied, not shared, when it's bound into the task. From an overhead
perspective, we take a small hit, since we have to keep around a WeakPtr and a
WeakPtrFactory.

Once this lands though, I have a patch to reduce the overhead of WeakPtr a
little bit, and I will also explore the feasibility of making it safe to call
GetWeakPtr() on multiple threads if there are no explicit InvalidateWeakPtr()
calls.

[dcheng, 2016-09-13 22:42:30]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2016-09-13 22:42:56]

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, 2016-09-14 00:47:24]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2016-09-14 00:50:04]

Description was changed from

==========
Implement WTF::WeakPtr in terms of base::WeakPtr

BUG=645774
==========

to

==========
Implement WTF::WeakPtr in terms of base::WeakPtr

BUG=645774

Committed: https://crrev.com/24430ece16aa10aa57644daeddafe25032fd359c
Cr-Commit-Position: refs/heads/master@{#418441}
==========

[commit-bot: I haz the power, 2016-09-14 00:50:05]

Patchset 2 (id:??) landed as
https://crrev.com/24430ece16aa10aa57644daeddafe25032fd359c
Cr-Commit-Position: refs/heads/master@{#418441}