Fix all -Wnon-virtual-dtor warnings.

This is needed to get the GN build going for several parts
of the code tree.

BUG=webrtc:3307
NOTRY=True
R=henrika@webrtc.org, nisse@webrtc.org

Review URL: https://codereview.webrtc.org/1928653005 .

Cr-Commit-Position: refs/heads/master@{#12693}

webrtc/api/api.gyp

@@ -66,7 +66,6 @@
             '-Wextra',
           ],
           'cflags_cc!': [
-            '-Wnon-virtual-dtor',
             '-Woverloaded-virtual',
           ],
           'msvs_disabled_warnings': [
@@ -207,7 +206,6 @@
         '-Wno-sign-compare',
       ],
       'cflags_cc!': [
-        '-Wnon-virtual-dtor',
         '-Woverloaded-virtual',
       ],
       'conditions': [