Expose useThreadedParserForDataUrls temporarily

Expose useThreadedParserForDataUrls temporarily

Exposing this to Chromium and having Chromium set it to
false when running browser_tests will make the transition
easier.

I attempted to delete this setting last week and at the same time
flip its value from false to true.  Unfortunately many browser_tests
depend on data urls being sync, despite the web at large not
depending on that behavior (since both IE and FF parse data urls async).

It's silly to expose this to chromium just to delete it a few days later
but unfortunately it's difficult to test blink changes through the chromium
try bots so making it possible for Chromium to control this setting
will make it much easier to test chromium-side fixes to browser_tests
in preparation for later removing this setting and making data urls
always async.

BUG=308321
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=172921

[abarth-chromium, 2014-04-29 18:18:34]

> Unfortunately many browser_tests depend on data urls being async

sync

[abarth-chromium, 2014-04-29 18:18:57]

lgtm

[eseidel, 2014-04-29 18:27:34]

The CQ bit was checked by eseidel@chromium.org

[eseidel, 2014-04-29 18:27:54]

The CQ bit was unchecked by eseidel@chromium.org

[eseidel, 2014-04-29 18:27:54]

The CQ bit was checked by eseidel@chromium.org

[commit-bot: I haz the power, 2014-04-29 18:28:14]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/eseidel@chromium.org/253823003/1

[commit-bot: I haz the power, 2014-04-30 04:31:12]

Change committed as 172921