Add GM/slide to simulate Android-style reveal clip

Hopefully, this will let us play w/ geometric and shader-based solutions.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2316593003

Committed: https://skia.googlesource.com/skia/+/ffac5c4aae18fc706e4077763c190a89c8507fb0
Review-Url: https://codereview.chromium.org/2316593003
diff --git a/gm/gm.h b/gm/gm.h
index e00a650..ff033a8 100644
--- a/gm/gm.h
+++ b/gm/gm.h
@@ -100,6 +100,9 @@
         }
 
         bool animate(const SkAnimTimer&);
+        bool handleKey(SkUnichar uni) {
+            return this->onHandleKey(uni);
+        }
 
         virtual void modifyGrContextOptions(GrContextOptions* options) {}
 
@@ -114,6 +117,7 @@
         virtual SkString onShortName() = 0;
 
         virtual bool onAnimate(const SkAnimTimer&) { return false; }
+        virtual bool onHandleKey(SkUnichar uni) { return false; }
         virtual SkMatrix onGetInitialTransform() const { return SkMatrix::I(); }
 
     private: