Enable whole-program virtual function optimization in LTO mode.

build/config/sanitizers/sanitizers.gni

Index: build/config/sanitizers/sanitizers.gni
diff --git a/build/config/sanitizers/sanitizers.gni b/build/config/sanitizers/sanitizers.gni
index 032380780236e6d23471a184a5ad34b009da1129..cd1865afa2bff022f196d745684a8f2975cbe096 100644
--- a/build/config/sanitizers/sanitizers.gni
+++ b/build/config/sanitizers/sanitizers.gni
@@ -39,6 +39,8 @@ declare_args() {
 
   # 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
@@ -68,10 +70,6 @@ declare_args() {
   use_custom_libcxx =
       (is_asan && is_linux) || is_tsan || is_msan || is_ubsan || 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
 }