viewer: Add a "Path renderer" dropdown menu

BUG=skia:

Change-Id: Ia3ed812d24f0f83631ab238bc418a3c95d49b9dc
Reviewed-on: https://skia-review.googlesource.com/9000
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
diff --git a/tools/viewer/sk_app/Window.h b/tools/viewer/sk_app/Window.h
index 4ae23b1..85cf3ac 100644
--- a/tools/viewer/sk_app/Window.h
+++ b/tools/viewer/sk_app/Window.h
@@ -14,6 +14,7 @@
 #include "SkTypes.h"
 #include "SkJSONCPP.h"
 
+class GrContext;
 class SkCanvas;
 class SkSurface;
 
@@ -198,6 +199,9 @@
     int sampleCount() const;
     int stencilBits() const;
 
+    // Returns null if there is not a GPU backend or if the backend is not yet created.
+    const GrContext* getGrContext() const;
+
 protected:
     Window();