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/SampleAtlas.cpp b/samplecode/SampleAtlas.cpp
index 7cf1bb3..3daf312 100644
--- a/samplecode/SampleAtlas.cpp
+++ b/samplecode/SampleAtlas.cpp
@@ -232,9 +232,11 @@
 
     void onDrawContent(SkCanvas* canvas) override {
         canvas->drawDrawable(fDrawable);
-        this->inval(nullptr);
     }
 
+    bool onAnimate(const SkAnimTimer&) override {
+        return true;
+    }
 #if 0
     // TODO: switch over to use this for our animation
     bool onAnimate(const SkAnimTimer& timer) override {