Enable ClangTidy flag modernize-use-override.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-override.html

Adds override (introduced in C++11) to overridden virtual functions and
removes virtual from those functions as it is not required.

virtual on non base class implementations was used to help indicate to
the user that a function was virtual. C++ compilers did not use the
presence of this to signify an overridden function.

Change-Id: If66d8919358f72a4035190caf8d7569268037a9a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310160
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/tools/gpu/mock/MockTestContext.cpp b/tools/gpu/mock/MockTestContext.cpp
index c413efb..06bd974 100644
--- a/tools/gpu/mock/MockTestContext.cpp
+++ b/tools/gpu/mock/MockTestContext.cpp
@@ -19,7 +19,7 @@
     MockTestContext() {}
     ~MockTestContext() override {}
 
-    virtual GrBackendApi backend() override { return GrBackendApi::kMock; }
+    GrBackendApi backend() override { return GrBackendApi::kMock; }
 
     void testAbandon() override {}
     void finish() override {}