Revert "Revert "Add method to sk_gpu_test::TestContext to automatically restore the previous context.""

This reverts commit 1e09e461d2ffcf8b07242cfe93dd7d12c4d75866.

Change-Id: I95d5544a7baaa078536790493ce4119816a77e94
Reviewed-on: https://skia-review.googlesource.com/72903
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/tools/gpu/gl/null/NullGLTestContext.cpp b/tools/gpu/gl/null/NullGLTestContext.cpp
index 894de07..9e7279b 100644
--- a/tools/gpu/gl/null/NullGLTestContext.cpp
+++ b/tools/gpu/gl/null/NullGLTestContext.cpp
@@ -22,6 +22,7 @@
 
 private:
     void onPlatformMakeCurrent() const override {}
+    std::function<void()> onPlatformGetAutoContextRestore() const override { return nullptr; }
     void onPlatformSwapBuffers() const override {}
     GrGLFuncPtr onPlatformGetProcAddress(const char*) const override { return nullptr; }
 };