Revert of Partial revert of Enable -Winconsistent-missing-override flag. (patchset #5 id:80001 of https://cod… (patchset #1 id:1 of https://codereview.webrtc.org/1944273002/ )

Reason for revert:
Downstream users updated now.

Original issue's description:
> Partial revert of Enable -Winconsistent-missing-override flag. (patchset #5 id:80001 of https://codereview.webrtc.org/1921653002/ )
>
> Reason for revert:
> This CL breaks the google3 import (but not the import bot).
> This partial revert only reverts the build files. A full revert no longer cleanly applies to ToT, so this was done instead.
>
> Original issue's description:
> > Enable -Winconsistent-missing-override flag.
> >
> > The problem with gmock is worked around by commenting out any other override declarations in classes using gmock.
> >
> > NOPRESUBMIT=True
> > BUG=webrtc:3970
> >
> > Committed: https://crrev.com/ef8b61e11062295365f11b9942f18a08a8b3ec60
> > Cr-Commit-Position: refs/heads/master@{#12563}
>
> TBR=mflodman@webrtc.org,kjellander@webrtc.org,nisse@webrtc.org
> BUG=webrtc:3970
>
> Committed: https://crrev.com/053f91774149a5367ddd531999d4ca69a57dbaa3
> Cr-Commit-Position: refs/heads/master@{#12624}

TBR=kjellander@webrtc.org,mflodman@webrtc.org,ivoc@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:3970
NOTRY=True

Review-Url: https://codereview.webrtc.org/1959353002
Cr-Commit-Position: refs/heads/master@{#12678}

webrtc/BUILD.gn

@@ -117,6 +117,7 @@ config("common_config") {
       cflags += [
         "-Wimplicit-fallthrough",
         "-Wthread-safety",
+        "-Winconsistent-missing-override",
       ]
     }
   }

webrtc/build/common.gypi

@@ -313,6 +313,7 @@
             'cflags': [
               '-Wimplicit-fallthrough',
               '-Wthread-safety',
+              '-Winconsistent-missing-override',
             ],
           }],
         ],