Add <chrome>/src/third_party dir to PYTHONPATH for Chrome checkouts.
Add <chrome>/src/third_party dir to PYTHONPATH for Chrome checkouts.

If chromite is living inside the Chrome checkout under
<chrome_root>/src/third_party/chromite, its dependencies will be
checked out to <chrome_root>/src/third_party instead of the normal
chromite/third_party location due to git-submodule limitations (a
submodule cannot be contained inside another submodule's workspace),
so we want to add that to the search path.

BUG=None
TEST=Local

Change-Id: I10a12bcddc88e509c1c7015a95c54d578fb8b122
Reviewed-on: https://gerrit.chromium.org/gerrit/43066
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

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