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..b75e92952f1afff6c98871bf1c37c7fc8ff9a713 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 LTCG. Only affects the VS
+  # Windows build.
+  use_incremental_ltcg = false

[brucedawson, 2016/11/03 18:13:19]

One could argue that this should be named "use_incremental_wpo" for consistency
with the full_wpo_on_official flag. But, I can see a case for either name, so it
is your call.

[Sébastien Marchand, 2016/11/03 21:18:02]

tough call, I'll go with the wpo variant as these 2 flags will almost always be
used together.

 }
 
 declare_args() {