Linux: initialize RenderSandboxHostLinux before threads are started.

RenderSandboxHostLinux::Init() uses fork(). Therefore the process needs
to only have one thread when this is called.

Move RenderSandboxHostLinux early enough that on one thread exists.

BUG=321388
R=creis@chromium.org, kbr@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/77123002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236651 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236651