Remove the Mac-specific implementation of LaunchApp, and share the Linux version.

Fixes a race condition with file descriptors, and gives the Mac access to the environment-alterning version of LaunchApp

BUG=11174
TEST=Launching render/plugin/utility processes should still work on the Mac

Review URL: http://codereview.chromium.org/165067

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@22649 0039d316-1c4b-4281-b951-d872f2087c98

• base/process_util.h[diff]
• base/process_util_linux.cc[diff]
• base/process_util_mac.mm[diff]
• base/process_util_posix.cc[diff]