chromium doesn't start anymore

chromium-browser: symbol lookup error: chromium-browser: undefined symbol: ZN3re23RE27ReplaceEPNSt7_cxx1112basic_stringIcSt11char_traitsIcESaIcEEERKS0_RKNS_11StringPieceE

GCC 5 uses the new C** ABI by default. When building new code, you might run
into link time errors that include lines similar to:
…: undefined reference to ‘_ZNSt6chrono12steady_clock3nowEv@GLIBCXX_3.4.17’

Or you might see linkage failures with “std::__cxx11::string” in the output.

These are signs that you need to rebuild packages using the new C** ABI.
You can quickly do so by using revdep-rebuild (from gentoolkit).

For gentoolkit-0.3.1 or higher:

 # revdep-rebuild --library '' -- --exclude gcc

For previous versions of gentoolkit:

 # revdep-rebuild --library 'libstdc\+\+\.so\.6' -- --exclude gcc

Now the update should fix the packages.