TSANv2: disable non sandbox-related threading checks under TSAN.

Allow Chromium to start under TSAN by removing the threading checks
before fork() in that environment.

Note that TSANv2 doesn't reliably support non-spawn fork() for now:
https://code.google.com/p/thread-sanitizer/issues/detail?id=44

But at least this allows using it on Chrome despite risks of running
into deadlocks.

NOTRY=true

Review URL: https://codereview.chromium.org/130423014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247708 0039d316-1c4b-4281-b951-d872f2087c98
Acquire lock in Args::SetSystemVars()

Since the function manipulates Args::declared_arguments_, it needs to
hold the lock to avoid heap corruption.

BUG=335587

Review URL: https://codereview.chromium.org/136783005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247710 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247710