Enable full WPO for the PGO builds.

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 11735ef2bac7bd12d0f5b484537614f8c2a1c3f7..88ea3d2b0fc1f40866da2785102a49fe880ba317 100644
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -20,6 +20,15 @@ declare_args() {
   # example, don't omit the frame pointer and leave in symbols.
   enable_profiling = false
 
+  # Specify the current PGO phase, only used for the Windows MSVS build. Here's
+  # the different values that can be used:
+  #     0 : Means that PGO is turned off.
+  #     1 : Used during the PGI (instrumentation) phase.
+  #     2 : Used during the PGO (optimization) phase.
+  #
+  # TODO(sebmarchand): Add support for the PGU (update) phase.
+  chrome_pgo_phase = 0
+
   # Whether or not the official builds should be build with full WPO.
   full_wpo_on_official = false
 }
@@ -47,3 +56,8 @@ if (symbol_level == -1) {
     symbol_level = 0
   }
 }
+
+# PGO requires full WPO.
+if (chrome_pgo_phase > 0) {
+  full_wpo_on_official = true
+}