Revert of Remove post_launch_on_client_thread_called

Revert of Remove post_launch_on_client_thread_called (patchset #2 id:20001 of https://codereview.chromium.org/2843063002/ )

Reason for revert:
Caused failure on WebKit Win10 builder:
webkit_python_tests webkit_python_tests
webkitpy/layout_tests/run_webkit_tests_unittest/RunTest/test_child_processes_2
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Win10/builds/21699
 

Original issue's description:
> Remove post_launch_on_client_thread_called
> 
> This is a follow up to refs/heads/master@{#465155}. Android now always
> calls OnChildProcessStarted on the launcher thread, which means the
> complexity to deal with the call directly happening on the client thread
> can be removed.
> 
> BUG=689758
> 
> Review-Url: https://codereview.chromium.org/2843063002
> Cr-Commit-Position: refs/heads/master@{#467463}
> Committed: https://chromium.googlesource.com/chromium/src/+/213830d7d68eefddedc6199bf699dfac3d5481e8

TBR=jcivelli@chromium.org,boliu@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=689758
Review-Url: https://codereview.chromium.org/2839353002
Cr-Commit-Position: refs/heads/master@{#467502}
Committed: https://chromium.googlesource.com/chromium/src/+/2582adf8aedbc7c9538aec544f80bf6379b11cc5

[sashab, 2017-04-26 23:17:00]

The CQ bit was checked by sashab@chromium.org

[sashab, 2017-04-26 23:17:00]

Created Revert of Remove post_launch_on_client_thread_called

[commit-bot: I haz the power, 2017-04-26 23:18:09]

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, 2017-04-26 23:18:34]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1493248620453820, "parent_rev":
"7dad4681ebb1ef820a076c8391fa16ff1afa21c0", "commit_rev":
"2582adf8aedbc7c9538aec544f80bf6379b11cc5"}

[commit-bot: I haz the power, 2017-04-26 23:18:45]

Description was changed from

==========
Revert of Remove post_launch_on_client_thread_called (patchset #2 id:20001 of
https://codereview.chromium.org/2843063002/ )

Reason for revert:
Caused failure on WebKit Win10 builder:
webkit_python_tests webkit_python_tests
webkitpy/layout_tests/run_webkit_tests_unittest/RunTest/test_child_processes_2
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Win1...
 

Original issue's description:
> Remove post_launch_on_client_thread_called
> 
> This is a follow up to refs/heads/master@{#465155}. Android now always
> calls OnChildProcessStarted on the launcher thread, which means the
> complexity to deal with the call directly happening on the client thread
> can be removed.
> 
> BUG=689758
> 
> Review-Url: https://codereview.chromium.org/2843063002
> Cr-Commit-Position: refs/heads/master@{#467463}
> Committed:
https://chromium.googlesource.com/chromium/src/+/213830d7d68eefddedc6199bf699...

TBR=jcivelli@chromium.org,boliu@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=689758
==========

to

==========
Revert of Remove post_launch_on_client_thread_called (patchset #2 id:20001 of
https://codereview.chromium.org/2843063002/ )

Reason for revert:
Caused failure on WebKit Win10 builder:
webkit_python_tests webkit_python_tests
webkitpy/layout_tests/run_webkit_tests_unittest/RunTest/test_child_processes_2
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Win1...
 

Original issue's description:
> Remove post_launch_on_client_thread_called
> 
> This is a follow up to refs/heads/master@{#465155}. Android now always
> calls OnChildProcessStarted on the launcher thread, which means the
> complexity to deal with the call directly happening on the client thread
> can be removed.
> 
> BUG=689758
> 
> Review-Url: https://codereview.chromium.org/2843063002
> Cr-Commit-Position: refs/heads/master@{#467463}
> Committed:
https://chromium.googlesource.com/chromium/src/+/213830d7d68eefddedc6199bf699...

TBR=jcivelli@chromium.org,boliu@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=689758

Review-Url: https://codereview.chromium.org/2839353002
Cr-Commit-Position: refs/heads/master@{#467502}
Committed:
https://chromium.googlesource.com/chromium/src/+/2582adf8aedbc7c9538aec544f80...
==========

[commit-bot: I haz the power, 2017-04-26 23:18:46]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/2582adf8aedbc7c9538aec544f80...

[boliu, 2017-04-26 23:22:30]

On 2017/04/26 23:18:46, commit-bot: I haz the power wrote:
> Committed patchset #1 (id:1) as
>
https://chromium.googlesource.com/chromium/src/+/2582adf8aedbc7c9538aec544f80...

Uhh... this change only had material effect on android

[boliu, 2017-04-26 23:25:27]

On 2017/04/26 23:22:30, boliu wrote:
> On 2017/04/26 23:18:46, commit-bot: I haz the power wrote:
> > Committed patchset #1 (id:1) as
> >
>
https://chromium.googlesource.com/chromium/src/+/2582adf8aedbc7c9538aec544f80...
> 
> Uhh... this change only had material effect on android

Next build already has green webkit_python_tests without this revert:
https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Win1...

relanding