GN: Add -Wimplicit-fallthrough warning

This warning was added for the GYP build back in
https://webrtc-codereview.appspot.com/34899004

NOTRY=True

Review URL: https://codereview.webrtc.org/1928543002

Cr-Commit-Position: refs/heads/master@{#12531}

webrtc/BUILD.gn

@@ -114,7 +114,10 @@ config("common_config") {
     }
 
     if (is_clang) {
-      cflags += [ "-Wthread-safety" ]
+      cflags += [
+        "-Wimplicit-fallthrough",
+        "-Wthread-safety",
+      ]
     }
   }