Add a flag to turn on incremental LTCG.

build/config/compiler/compiler.gni

Index: build/config/compiler/compiler.gni
diff --git a/build/config/compiler/compiler.gni b/build/config/compiler/compiler.gni
index fa097036c15e54a17835d6adfd992bada8760198..11a078ac33556493683a96e236744627937d4b0a 100644
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -48,6 +48,10 @@ declare_args() {
   #
   # TODO(sebmarchand): Add support for the PGU (update) phase.
   chrome_pgo_phase = 0
+
+  # Whether or not we should turn on incremental WPO. Only affects the VS
+  # Windows build.
+  use_incremental_wpo = false
 }
 
 declare_args() {