Disable warning 4702: Unreachable code for the WPO/PGO builds.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 5e1e024ea5ff15eebdb3307d77058d67673073c9..2d3b498d48d251855bc8b1bd3abf7ea3467da4f7 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1246,7 +1246,15 @@ if (is_win) {
 config("optimize") {
   if (is_win) {
     if (is_official_build && full_wpo_on_official) {
-      common_optimize_on_cflags += [ "/GL" ]
+      common_optimize_on_cflags += [
+        "/GL",  # Whole program optimization.
+
+        # Disable Warning 4702 ("Unreachable code") for the WPO/PGO builds.
+        # Probably anything that this would catch that wouldn't be caught in a
+        # normal build isn't going to actually be a bug, so the incremental
+        # value of C4702 for PGO builds is likely very small.
+        "/wd4702",
+      ]
     }
 
     # Favor size over speed, /O1 must be before the common flags. The GYP