Disable Warning 4702 for the PGO builds.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 5a2753f08af3172e7c9d25e2efb04e6479d74aad..fe22c66aa14abb0b2cec5e4f9028859afd5717fe 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -4967,6 +4967,14 @@
                   },
                 ],
                 ['optimize=="max"', {
+                    # 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.
+                    'msvs_disabled_warnings': [
+                      4702
+                    ],
                     'msvs_settings': {
                       'VCCLCompilerTool': {
                         # 2, optimizeMaxSpeed, Maximize Speed (/O2)