fix excesive number of mutexes created by sandbox

The |cient_control->server_alive| mutex was being created for each
target (child) process and leaked (that part is necessary). Instead
a single mutex can be reused for all targets.

R=rvargas
BUG=331241

Review URL: https://codereview.chromium.org/167593003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252782 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252782
Add content implementation of digestSynchcronous.

BUG=327826

Review URL: https://codereview.chromium.org/138343008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252783 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252783