Re-enabling -Woverloaded-virtual.

We should build with this flag; some of our downstream users do. This
was found necessary after a recent breakage was introduced with
https://codereview.webrtc.org/1972793003.

Verified by hand that this would have caught the above error.

R=kjellander@webrtc.org
CC=deadbeef@webrtc.org

Review URL: https://codereview.webrtc.org/2035593003 .

Cr-Commit-Position: refs/heads/master@{#13013}

webrtc/api/api.gyp

@@ -67,9 +67,6 @@
           'cflags!': [
             '-Wextra',
           ],
-          'cflags_cc!': [
-            '-Woverloaded-virtual',
-          ],
           'msvs_disabled_warnings': [
             4245,  # conversion from 'int' to 'size_t', signed/unsigned mismatch.
             4267,  # conversion from 'size_t' to 'int', possible loss of data.
@@ -207,9 +204,6 @@
       'cflags': [
         '-Wno-sign-compare',
       ],
-      'cflags_cc!': [
-        '-Woverloaded-virtual',
-      ],
       'conditions': [
         ['clang==1', {
           'cflags!': [