[GN] Make |use_system_harfbuzz| configurable

[GN] Make |use_system_harfbuzz| configurable

Fixes link error on building chrome for bundled harfbuzz on Ubuntu 14.04

BUG=None

Committed: https://crrev.com/01d0d071840dc49d517289eddb38aafb93676a07
Cr-Commit-Position: refs/heads/master@{#293989}

[tzik, 2014-09-09 03:57:34]

tzik@chromium.org changed reviewers:
+ brettw@chromium.org

[tzik, 2014-09-09 04:06:02]

Hi, Brett. PTL.

Chrome does not build on my Ubuntu 14.04 due to an link error.
The error message was same to http://crbug.com/353127.
And setting |use_system_harfbuzz| to true works fine.

Can I make it configurable?

[brettw, 2014-09-09 17:39:20]

lgtm

[tzik, 2014-09-09 18:14:21]

The CQ bit was checked by tzik@chromium.org

[commit-bot: I haz the power, 2014-09-09 18:21:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tzik@chromium.org/547283004/1

[commit-bot: I haz the power, 2014-09-09 19:39:55]

Committed patchset #1 (id:1) as c74e632727ed1ae6855e58618c53f66b0c28f3ca

[commit-bot: I haz the power, 2014-09-10 03:55:00]

Patchset 1 (id:??) landed as
https://crrev.com/01d0d071840dc49d517289eddb38aafb93676a07
Cr-Commit-Position: refs/heads/master@{#293989}