Allow C++ exceptions on shill. shill uses try/catch to interact with dbus-c++. BUG=chromium:386443 TEST=emerge-link platform2 (with the changes to common.gypi) CQ-DEPEND=CL:205242 Change-Id: I2b316da69d952b34eac63b8ba9b6a0494c97183c Reviewed-on: https://chromium-review.googlesource.com/205232 Reviewed-by: mukesh agrawal <quiche@chromium.org> Commit-Queue: Alex Deymo <deymo@chromium.org> Tested-by: Alex Deymo <deymo@chromium.org>