This got changed by Gentoo maintainers, who have started putting the most short-living patches in separate git repositories, which many other distros have similarly started to make use of. Most distros, including Alpine (maintained by me), Arch, Gentoo, and Fedora, started building the whole Chromium with the source code of LLVM's libc++ that Chromium is shipped with by default. I've patched Chromium enough to build with GCC and libstdc++.