Add a DCHECK to make sure that SpawnTarget is not called from multiple threads.
Add a DCHECK to make sure that SpawnTarget is not called
from multiple threads.

In chrome all child processes are started from the
PROCESS_LAUNCHER thread.

BUG=28798
Review URL: http://codereview.chromium.org/548192

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@37712 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser