Allow C++ exceptions on debugd.

debugd 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: Ia176d850895ae5156a67973ec08f1bab9dcedde1
Reviewed-on: https://chromium-review.googlesource.com/205233
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>