Disable Warning 4702 for the 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 4520f3aef2056534e5cca154ca5d2a16d7c22420..f9d966bd69e11a2094a2d822fff128cadf4dc6de 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -767,6 +767,11 @@ config("optimize_max") {
     cflags += [
       "/Ot",   # Favor speed over size.
       "/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",
     ]
   } else {
     cflags += [