Fix VideoDetectorTest clang tidy

On a separate CL [1] Tricium found a case where a trivial destructor
was {} but should have been =default. Since this change doesn't belong
in that CL, I created a separate CL.

This CL simply changes a {} destructor to =default.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1978634/

Bug: 955158
Change-Id: Ide358b8f64b0084f5615221241e0ce40f5ce21ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013622Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733781}

components/viz/service/frame_sinks/video_detector_unittest.cc

@@ -84,7 +84,7 @@ class VideoDetectorTest : public testing::Test {
                             false,
                             false) {}
 
-  ~VideoDetectorTest() override {}
+  ~VideoDetectorTest() override = default;
 
   void SetUp() override {
     mock_task_runner_ = base::MakeRefCounted<base::TestMockTimeTaskRunner>(