Do not use std::set::at in Config.cpp.
Do not use std::set::at in Config.cpp.

This is C++11 STL.

BUG=459399

Change-Id: I2e994e6b47535bcb38c193db2aaecc304e388772
Reviewed-on: https://chromium-review.googlesource.com/250581
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

larsbergstrom/angle,vvuk/angle,mikolalysenko/angle,mikolalysenko/angle,mrobinson/rust-angle,ghostoy/angle,vvuk/angle,MSOpenTech/angle,csa7mdm/angle,ppy/angle,mikolalysenko/angle,vvuk/angle,ecoal95/angle,ppy/angle,ecoal95/angle,vvuk/angle,mrobinson/rust-angle,ghostoy/angle,crezefire/angle,csa7mdm/angle,mybios/angle,ghostoy/angle,mikolalysenko/angle,bsergean/angle,mrobinson/rust-angle,mlfarrell/angle,mybios/angle,mybios/angle,crezefire/angle,ecoal95/angle,crezefire/angle,mrobinson/rust-angle,MSOpenTech/angle,bsergean/angle,mrobinson/rust-angle,MSOpenTech/angle,csa7mdm/angle,bsergean/angle,larsbergstrom/angle,crezefire/angle,domokit/waterfall,ppy/angle,mlfarrell/angle,larsbergstrom/angle,ecoal95/angle,ecoal95/angle,domokit/waterfall,csa7mdm/angle,ghostoy/angle,bsergean/angle,mlfarrell/angle,ppy/angle,mybios/angle,larsbergstrom/angle,MSOpenTech/angle,mlfarrell/angle