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

BUG=490934

Committed: https://crrev.com/04269068016b6ee7ac16c1d93c118668ac3a8936
Cr-Commit-Position: refs/heads/master@{#396647}

Review-Url: https://codereview.chromium.org/2023653002
Cr-Original-Commit-Position: refs/heads/master@{#396659}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 70bc55033e16ed4696ccbaf7e091d426d538bd08

common.gypi

@@ -5546,6 +5546,16 @@
               4702, # unreachable code
             ],
           }],
+          ['full_wpo_on_official==1', {
+            # 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
+            ],
+          }],
         ],
         # Add extra include directories here that need to be in front of the
         # installed and packaged include directories. This may be needed in

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