Fix warnings caused by gmock on linux

src/libANGLE/renderer/ImageImpl_mock.h

@@ -18,7 +18,7 @@ namespace rx
 class MockImageImpl : public ImageImpl
 {
   public:
-    ~MockImageImpl() override { destructor(); }
+    virtual ~MockImageImpl() { destructor(); }
     MOCK_METHOD0(initialize, egl::Error(void));
     MOCK_METHOD1(orphan, gl::Error(egl::ImageSibling *));
     MOCK_METHOD0(destructor, void());

src/libANGLE/renderer/RenderbufferImpl_mock.h

@@ -20,7 +20,7 @@ namespace rx
 class MockRenderbufferImpl : public RenderbufferImpl
 {
   public:
-    ~MockRenderbufferImpl() override { destructor(); }
+    virtual ~MockRenderbufferImpl() { destructor(); }
     MOCK_METHOD3(setStorage, gl::Error(GLenum, size_t, size_t));
     MOCK_METHOD4(setStorageMultisample, gl::Error(size_t, GLenum, size_t, size_t));
     MOCK_METHOD1(setStorageEGLImageTarget, gl::Error(egl::Image *));

src/libANGLE/renderer/TextureImpl_mock.h

@@ -19,7 +19,7 @@ namespace rx
 class MockTextureImpl : public TextureImpl
 {
   public:
-    ~MockTextureImpl() override { destructor(); }
+    virtual ~MockTextureImpl() { destructor(); }
     MOCK_METHOD1(setUsage, void(GLenum));
     MOCK_METHOD8(setImage, gl::Error(GLenum, size_t, GLenum, const gl::Extents &, GLenum, GLenum, const gl::PixelUnpackState &, const uint8_t *));
     MOCK_METHOD7(setSubImage, gl::Error(GLenum, size_t, const gl::Box &, GLenum, GLenum, const gl::PixelUnpackState &, const uint8_t *));