We noticed that builds stopped working on Friday afternoon with this in the log:

[12153/35710] CC host/obj/third_party/yasm/yasm/symrec.o
[12154/35710] LINK host/transport_security_state_generator
FAILED: host/transport_security_state_generator 
python "../../../../../../../home/pdswauto/chrome_root/src/build/toolchain/gcc_link_wrapper.py" --output="host/transport_security_state_generator" -- x86_64-pc-linux-gnu-g++ -Wl,--fatal-warnings -fPIC -Wl,-z,noexecstack -Wl,-z,now -Wl,-z,relro -Wl,-z,defs -Wl,--no-as-needed -lpthread -Wl,--as-needed -fuse-ld=gold -B../../../../../../../home/pdswauto/chrome_root/src/third_party/binutils/Linux_x64/Release/bin -Wl,--icf=all -m64 -pthread -Wl,-O1 -Wl,--gc-sections -Wl,--gdb-index -Wl,-rpath-link=host -Wl,--disable-new-dtags -o "host/transport_security_state_generator" -Wl,--start-group @"host/transport_security_state_generator.rsp"  -Wl,--end-group  -ldl -lrt -latomic -lsmime3 -lnss3 -lnssutil3 -lplds4 -lplc4 -lnspr4 
../../../../../../../home/pdswauto/chrome_root/src/third_party/binutils/Linux_x64/Release/bin/ld.gold: error: cannot find -latomic
collect2: error: ld returned 1 exit status

It's not clear where the -latomic library requirement comes from. A workaround is to do 

  USE="go" sudo emerge sys-devel/gcc

before using cros_workon_make to compile chrome. This is because USE="go" will add --enable-libatomic to the configure options inside the gcc-4.9.2-r152.ebuild.