P2: Important I can only reproduce this in dev branch. 5.12 works fine. Release build in dev also works.
The error is:
[12/5100] /usr/bin/python2 "../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/build/toolchain/gcc_link_wrapper.py" --output="host/top_domain_generator" -- /usr/bin/g++ -Wl,--fatal-warnings -fPIC -Wl,-z,noexecstack -Wl,-z,now -Wl,-z,relro -Wl,-z,defs -Wl,--as-needed -fuse-ld=gold -B../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/third_party/binutils/Linux_x64/Release/bin -m64 -Wl,--gdb-index -rdynamic -Wl,-rpath-link=host -Wl,--disable-new-dtags -o "host/top_domain_generator" -Wl,--start-group @"host/top_domain_generator.rsp" -Wl,--end-group -ldl -lpthread -lrt FAILED: host/top_domain_generator /usr/bin/python2 "../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/build/toolchain/gcc_link_wrapper.py" --output="host/top_domain_generator" -- /usr/bin/g++ -Wl,--fatal-warnings -fPIC -Wl,-z,noexecstack -Wl,-z,now -Wl,-z,relro -Wl,-z,defs -Wl,--as-needed -fuse-ld=gold -B../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/third_party/binutils/Linux_x64/Release/bin -m64 -Wl,--gdb-index -rdynamic -Wl,-rpath-link=host -Wl,--disable-new-dtags -o "host/top_domain_generator" -Wl,--start-group @"host/top_domain_generator.rsp" -Wl,--end-group -ldl -lpthread -lrt ../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/components/url_formatter/top_domains/top_domain_generator.cc:74: error: undefined reference to 'base::BasicStringPiece<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::BasicStringPiece(char const*)' ../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/components/url_formatter/top_domains/top_domain_generator.cc:87: error: undefined reference to 'base::BasicStringPiece<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::BasicStringPiece(char const*)' ../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/components/url_formatter/top_domains/top_domain_generator.cc:99: error: undefined reference to 'base::BasicStringPiece<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::BasicStringPiece(char const*)' ../../../../../qt5-dev-src/qtwebengine/src/3rdparty/chromium/components/url_formatter/top_domains/top_domain_generator.cc:121: error: undefined reference to 'base::BasicStringPiece<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::BasicStringPiece(char const*)' collect2: error: ld returned 1 exit status
It seems the implicit conversion of char const* to std::string doesn't work. Converting the char* to std::string explicitly at the code lines fixes the error but this can't be a solution and also there are too many of them.
I also tried to build dev branch of QtWebEngine with Qt 5.12 and it worked. I'm not sure whether the issue is in QtWebEngine.