cbuildbot - upload chrome sysroot for each build.
cbuildbot - upload chrome sysroot for each build.

Cbuildbot calls the 'cros_generate_sysroot' script to generate a sysroot
for Chrome, and archives the resulting tarball.

BUG=chromium-os:36299
TEST=unit tests, local trybot, remote trybot.

Change-Id: I5ee55d44f97324c2d4c99f93ad002e017e884aae
Reviewed-on: https://gerrit.chromium.org/gerrit/38855
Commit-Ready: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite