Add an option to share the cache data with other one.

Test: remove the default cache path, run a cached test, it shows cache
      miss.
      run ./crosperf --share_users=yunlian, it shows cache hit

PRESUBMIT=passed
R=llozano,cmtice,asharif
APPROVED=llozano
DELTA=33  (19 added, 0 deleted, 14 changed)
OCL=65354-p2
RCL=65672-p2
RDATE=2013/02/05 15:53:27


P4 change: 42807223
6 files changed
tree: 569c34dabc4cc80ae4cbc9b30dc170ff84214361
  1. .git5_specs/
  2. v14/