shill: re-enable unused-result warning Eliminate code that violates no-unused-result, and remove -Wno-unused-result from the c++ compiler flags in the gyp file. BUG=chromium:293668 TEST=unit tests Change-Id: I2edb193e0cf43c2ca70545ada11229a1737f5d0c Reviewed-on: https://chromium-review.googlesource.com/195763 Tested-by: mukesh agrawal <quiche@chromium.org> Reviewed-by: Ben Chan <benchan@chromium.org> Commit-Queue: mukesh agrawal <quiche@chromium.org>