Viewer to resize Samples with window resize.

This adds onResize to sk_app::Window::Layer and uses it to resize
samples in Viewer.

Change-Id: Ia5ec5cf6dc74fbdf712291b9ba8707b076cb625d
Reviewed-on: https://skia-review.googlesource.com/146642
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/tools/viewer/Slide.h b/tools/viewer/Slide.h
index 127b32d..023cd6b 100644
--- a/tools/viewer/Slide.h
+++ b/tools/viewer/Slide.h
@@ -28,6 +28,7 @@
     virtual void draw(SkCanvas* canvas) = 0;
     virtual bool animate(const SkAnimTimer&) { return false;  }
     virtual void load(SkScalar winWidth, SkScalar winHeight) {}
+    virtual void resize(SkScalar winWidth, SkScalar winHeight) {}
     virtual void unload() {}
 
     virtual bool onChar(SkUnichar c) { return false; }