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}
diff --git a/webrtc/api/api.gyp b/webrtc/api/api.gyp
index e705831..377ef8f 100644
--- a/webrtc/api/api.gyp
+++ b/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': [
diff --git a/webrtc/api/api_tests.gyp b/webrtc/api/api_tests.gyp
index 4434420..65bb461 100644
--- a/webrtc/api/api_tests.gyp
+++ b/webrtc/api/api_tests.gyp
@@ -72,7 +72,6 @@
         '-Wextra',
       ],
       'cflags_cc!': [
-        '-Wnon-virtual-dtor',
         '-Woverloaded-virtual',
       ],
       'msvs_disabled_warnings': [
diff --git a/webrtc/api/peerconnectionfactory_unittest.cc b/webrtc/api/peerconnectionfactory_unittest.cc
index e7859d0..833242a 100644
--- a/webrtc/api/peerconnectionfactory_unittest.cc
+++ b/webrtc/api/peerconnectionfactory_unittest.cc
@@ -64,6 +64,7 @@
 
 class NullPeerConnectionObserver : public PeerConnectionObserver {
  public:
+  virtual ~NullPeerConnectionObserver() = default;
   virtual void OnMessage(const std::string& msg) {}
   virtual void OnSignalingMessage(const std::string& msg) {}
   virtual void OnSignalingChange(
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index 2594b6c..1386e8c 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -416,7 +416,7 @@
 class MockPeerConnectionObserver : public PeerConnectionObserver {
  public:
   MockPeerConnectionObserver() : remote_streams_(StreamCollection::Create()) {}
-  ~MockPeerConnectionObserver() {
+  virtual ~MockPeerConnectionObserver() {
   }
   void SetPeerConnectionInterface(PeerConnectionInterface* pc) {
     pc_ = pc;
diff --git a/webrtc/api/webrtcsession_unittest.cc b/webrtc/api/webrtcsession_unittest.cc
index 87481a8..45ffc36 100644
--- a/webrtc/api/webrtcsession_unittest.cc
+++ b/webrtc/api/webrtcsession_unittest.cc
@@ -158,6 +158,8 @@
         ice_gathering_state_(PeerConnectionInterface::kIceGatheringNew) {
   }
 
+  virtual ~MockIceObserver() = default;
+
   void OnIceConnectionChange(
       PeerConnectionInterface::IceConnectionState new_state) override {
     ice_connection_state_ = new_state;