Make ShareGroup thread safe

In another CL I'll refactor the id handlers AND
make the places sharing resources correctly set things
up.

TEST=none
BUG=120297


Review URL: http://codereview.chromium.org/9918027

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@130034 0039d316-1c4b-4281-b951-d872f2087c98
10 files changed