[Mac] Only create the App Shim Socket after taking process singleton lock.
Previously, Chrome processes initialized AppShimHostManager before taking
the singleton lock. This meant new processes would replace the App Shim
Socket symlink and then delete it as soon as they fail to become the
singleton process.
BUG=168080
TEST= Start Chrome.
Start Chrome again via the command line. I.e. Chromium.app/Contents/MacOS/Chromium
Start an app via its shim. It should work.
Review URL: https://codereview.chromium.org/
268903002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268469 0039d316-1c4b-4281-b951-d872f2087c98