Suppressing /wd4702.

Bug: webrtc:9274
Change-Id: I420a96202d9de323341ef7831bbf90f986958003
Reviewed-on: https://webrtc-review.googlesource.com/77040
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23261}

BUILD.gn

@@ -259,6 +259,13 @@ config("common_config") {
         cflags += [ "-Wunused-lambda-capture" ]
       }
     }
+
+    if (is_win && !is_clang) {
+      # MSVC warning suppressions (needed to use Abseil).
+      # TODO(bugs.webrtc.org/9274): Remove these warnings as soon as MSVC allows
+      # external headers warning suppression (or fix them upstream).
+      cflags += [ "/wd4702" ]  # unreachable code
+    }
   }
 
   if (current_cpu == "arm64") {