Include X11 headers only when using X11.

Use X11 headers only when using X11.

This was causing problems when building chromium on Linux platforms with no x11 and Ozone enabled. https://codereview.chromium.org/817023005/diff/40001/content/common/BUILD.gn
made changes so that we run the version in content_common with a va_stub_header.fragment always.

Committed: https://crrev.com/f51c0e9efe38e89b0172be3472bbf0517010642b
Cr-Commit-Position: refs/heads/master@{#312563}

[kalyank, 2015-01-21 06:55:17]

kalyan.kondapally@intel.com changed reviewers:
+ dalecurtis@chromium.org

[kalyank, 2015-01-21 17:49:27]

PTAL

[DaleCurtis, 2015-01-21 19:10:00]

lgtm

[kalyank, 2015-01-22 04:38:25]

The CQ bit was checked by kalyan.kondapally@intel.com

[commit-bot: I haz the power, 2015-01-22 04:38:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/860163003/1

[commit-bot: I haz the power, 2015-01-22 04:49:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-22 04:50:29]

Patchset 1 (id:??) landed as
https://crrev.com/f51c0e9efe38e89b0172be3472bbf0517010642b
Cr-Commit-Position: refs/heads/master@{#312563}