See: https://build.chromium.org/p/chromium.fyi/builders/Libfuzzer%20Upload%20Linux%20UBSan/builds/764/steps/compile/logs/stdio

../../third_party/llvm-build/Release+Asserts/bin/clang++ -Wl,--fatal-warnings -fPIC -Wl,-z,noexecstack -Wl,-z,now -Wl,-z,relro -fuse-ld=gold -B../../third_party/binutils/Linux_x64/Release/bin -pthread -m64 -Wl,-O1 -Wl,--gc-sections --sysroot=../../build/linux/debian_wheezy_amd64-sysroot -L/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/lib/x86_64-linux-gnu -Wl,-rpath-link=/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/lib/x86_64-linux-gnu -L/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/usr/lib/x86_64-linux-gnu -Wl,-rpath-link=/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/usr/lib/x86_64-linux-gnu -L/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/usr/lib/gcc/x86_64-linux-gnu/4.6 -Wl,-rpath-link=/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/usr/lib/gcc/x86_64-linux-gnu/4.6 -L/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/usr/lib -Wl,-rpath-link=/b/build/slave/Libfuzzer_Upload_Linux_UBSan/build/src/build/linux/debian_wheezy_amd64-sysroot/usr/lib -fsanitize=undefined -Wl,-rpath-link=../Release -Wl,--disable-new-dtags -Wl,-rpath=\$ORIGIN/. -Wl,-rpath-link=. -Wl,-u_sanitizer_options_link_helper -fsanitize=undefined -stdlib=libc++ -Wl,-R,\$ORIGIN/ -L. -o "./pdfium_fuzzer" -Wl,--start-group @"./pdfium_fuzzer.rsp" ./libc++.so -Wl,--end-group  -ldl -lrt -lfreetype 
obj/third_party/pdfium/libxfa.a(fwl_noteimp.o)(.data..L__unnamed_259+0x18): error: undefined reference to 'typeinfo for IFWL_ToolTipTarget'
obj/third_party/pdfium/libxfa.a(fwl_noteimp.o)(.data..L__unnamed_25+0x18): error: undefined reference to 'typeinfo for IFWL_ToolTipTarget'
obj/third_party/pdfium/libxfa.a(fwl_noteimp.o)(.data..L__unnamed_27+0x18): error: undefined reference to 'typeinfo for IFWL_ToolTipTarget'
obj/third_party/pdfium/libxfa.a(fwl_noteimp.o)(.data..L__unnamed_29+0x18): error: undefined reference to 'typeinfo for IFWL_ToolTipTarget'

Seems like it started happening after this roll: https://chromium.googlesource.com/chromium/src/+/f7014ec458435b22a73f4a085e50764444351ee9

IFWL_ToolTipTarget seems to a be an abstract class with no implementation..