Enable whole-program virtual function optimization in LTO mode.

build/config/sanitizers/sanitizers.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   # Compile for Address Sanitizer to find memory bugs.
   is_asan = false
 
   # Compile for Leak Sanitizer to find leaks.
   is_lsan = false
@@ skipping 21 matching lines @@
   # GCS.
   use_prebuilt_instrumented_libraries = false
 
   # Enable building with SyzyAsan which can find certain types of memory
   # errors. Only works on Windows. See
   # https://github.com/google/syzygy/wiki/SyzyASanHowTo
   is_syzyasan = false
 
   # Compile with Control Flow Integrity to protect virtual calls and casts.
   # See http://clang.llvm.org/docs/ControlFlowIntegrity.html
+  #
+  # TODO(pcc): Remove this flag if/when CFI is enabled in official builds.
   is_cfi = false
 
   # By default, Control Flow Integrity will crash the program if it detects a
   # violation. Set this to true to print detailed diagnostics instead.
   use_cfi_diag = false
 
   # Compile for fuzzing with LLVM LibFuzzer.
   # See http://www.chromium.org/developers/testing/libfuzzer
   use_libfuzzer = false
 
@@ skipping 13 matching lines @@
 # Args that are in turn dependent on other args must be in a separate
 # declare_args block. User overrides are only applied at the end of a
 # declare_args block.
 declare_args() {
   # Use libc++ (buildtools/third_party/libc++ and
   # buildtools/third_party/libc++abi) instead of stdlibc++ as standard library.
   # This is intended to be used for instrumented builds.
   use_custom_libcxx = (is_asan && is_linux) || is_tsan || is_msan || is_ubsan ||
                       is_ubsan_security || use_libfuzzer
 
-  # Enable Link Time Optimization (output programs runs faster,
-  # but linking is up to 5-20x slower.
-  is_lto = is_cfi
-
   use_sanitizer_coverage = use_libfuzzer
 }
 
 using_sanitizer = is_asan || is_lsan || is_tsan || is_msan || is_ubsan ||
                   is_ubsan_vptr || is_ubsan_security
 
 assert(!using_sanitizer || is_clang,
        "Sanitizers (is_*san) require setting is_clang = true in 'gn args'")
 
 # MSan only links Chrome properly in release builds (brettw -- 9/1/2015). The
 # same is possibly true for the other non-ASan sanitizers. But regardless of
 # whether it links, one would normally never run a sanitizer in debug mode.
 # Running in debug mode probably indicates you forgot to set the "is_debug =
 # false" flag in the build args. ASan seems to run fine in debug mode.
 #
 # If you find a use-case where you want to compile a sanitizer in debug mode
 # and have verified it works, ask brettw and we can consider removing it from
 # this condition. We may also be able to find another way to enable your case
 # without having people accidentally get broken builds by compiling an
 # unsupported or unadvisable configurations.
 #
 # For one-off testing, just comment this assertion out.
 assert(
     !is_debug || !(is_msan || is_lsan || is_tsan || is_ubsan || is_ubsan_vptr),
     "Sanitizers should generally be used in release (set is_debug=false).")