Revert of Disable warning 4702: Unreachable code for the WPO/PGO builds. (patchset #2 id:40001 of https://codereview.chromium.org/2023653002/ )

Reason for revert:
Broke DrMemory builders:

gyp: Key 'conditions' repeated at level 5 with key path
'conditions.12.1.target_defaults' while reading
C:\b\build\slave\drm-cr\build\src\build\common.gypi while reading
includes of C:\b\build\slave\drm-cr\build\src\build\all.gyp

Original issue's description:
> Disable warning 4702: Unreachable code for the WPO/PGO builds.
>
> BUG=490934
>
> Committed: https://crrev.com/04269068016b6ee7ac16c1d93c118668ac3a8936
> Cr-Commit-Position: refs/heads/master@{#396647}

TBR=scottmg@chromium.org,sebmarchand@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=490934

Review-Url: https://codereview.chromium.org/2023693003
Cr-Original-Commit-Position: refs/heads/master@{#396651}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7c271e2113631dee0fb917154d8ba5814d56e33b

common.gypi

@@ -5639,18 +5639,6 @@
           # See http://bugs.icu-project.org/trac/ticket/11122
           4595,
         ],
-        'conditions': [
-          ['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
-            ],
-          }],
-        ],
         'msvs_settings': {
           'VCCLCompilerTool': {
             'AdditionalOptions': ['/MP'],

config/compiler/BUILD.gn

@@ -1246,15 +1246,7 @@ if (is_win) {
 config("optimize") {
   if (is_win) {
     if (is_official_build && full_wpo_on_official) {
-      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",
-      ]
+      common_optimize_on_cflags += [ "/GL" ]
     }
 
     # Favor size over speed, /O1 must be before the common flags. The GYP