Protect reading and writing startup_data_ to ensure intruction order and
to ensure the value is correctly read in the new thread.

Contributed by phanna (Android)

BUG=67145
TEST=none

http://codereview.chromium.org/5933002/

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69438 0039d316-1c4b-4281-b951-d872f2087c98