Add samples to Viewer.

This adds support with animation, assuming the sample has
implemented onAnimate. Event handling has not been
implemented.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2056343004

Committed: https://skia.googlesource.com/skia/+/76963e73704a42a18c29d6fbdcccb566e5c67658
Review-Url: https://codereview.chromium.org/2056343004
diff --git a/samplecode/SampleClock.cpp b/samplecode/SampleClock.cpp
index ae7462d..ff3a5b1 100644
--- a/samplecode/SampleClock.cpp
+++ b/samplecode/SampleClock.cpp
@@ -215,8 +215,10 @@
 #endif
 
         canvas->restore();
+    }
 
-        this->inval(nullptr);
+    bool onAnimate(const SkAnimTimer&) override {
+        return true;
     }
 
 private: