Relanding 236651: https://codereview.chromium.org/77123002

Linux: initialize RenderSandboxHostLinux before threads are started.

A few tests in browser_tests seems to be relying on TestSuite in base/test/test_suite.cc, which is calling gtk_init. Instead of fixing gtk config, I simply skipped DCHECK on TOOLKIT_GTK as this config will be obsolete soon.

BUG=321388
R=creis@chromium.org, jln@chromium.org, kbr@chromium.org
TBR=creis@chromium.org, jln@chromium.org, kbr@chromium.org

Review URL: https://codereview.chromium.org/83813002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236805 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236805