Use a joinable background thread instead of worker pool in gin

Use a joinable background thread instead of worker pool in gin

gin's V8Platform wants to provide a way to run background tasks on V8's
behalf. It was using a base::WorkerPool, which lazily constructs non
joinable threads to execute tasks. This is problematic when running
inside a shared library that needs to stop completely so it can be
unloaded. This uses a single lazily constructed thread for background
tasks. In the context of js_content_handler there likely isn't another
thread sitting around which could share work with this since all the
app does is run JS, so pooling isn't that useful.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/74f86ec4260f259401b594ff6129306465a5a801

[jamesr, 2015-03-05 20:24:50]

jamesr@chromium.org changed reviewers:
+ msw@chromium.org

[abarth-chromium, 2015-03-05 20:43:19]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-03-05 20:43:19]

lgtm

[jamesr, 2015-03-05 21:07:13]

Committed patchset #1 (id:1) manually as
74f86ec4260f259401b594ff6129306465a5a801 (presubmit successful).

[msw, 2015-03-05 21:31:52]

Neat, it didn't occur to me to avoid base::WorkerPool from V8Platform. That
should solve most of the flaky crashes for <http://crbug.com/462755>, but we
still need to resolve the general issue you filed, <http://crbug.com/464506>,
right?

[jamesr, 2015-03-05 21:40:23]

On 2015/03/05 21:31:52, msw wrote:
> Neat, it didn't occur to me to avoid base::WorkerPool from V8Platform. That
> should solve most of the flaky crashes for <http://crbug.com/462755>, but we
> still need to resolve the general issue you filed, <http://crbug.com/464506>,
> right?

I think this will fix the particular crash in js_apptests since the js content
handler was only using base::WorkerPool for this particular thing.  We aren't
using base::WorkerPool for anything else within the mojo repository.

All mojo apps that do use this functionality (which today means the network
service, html_viewer, and possibly the media services) will need a resolution to
http://crbug.com/464506 in order to function properly.