Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/GLWindowContext.h

Index: tools/viewer/sk_app/GLWindowContext.h
diff --git a/tools/viewer/sk_app/GLWindowContext.h b/tools/viewer/sk_app/GLWindowContext.h
index acf15a332e15c8b44fb03b1763b2b0448d5166c4..e9abffd6d18c4712cad217dd3d8f02aa90b6ed6d 100644
--- a/tools/viewer/sk_app/GLWindowContext.h
+++ b/tools/viewer/sk_app/GLWindowContext.h
@@ -23,9 +23,6 @@ namespace sk_app {
 
 class GLWindowContext : public WindowContext {
 public:
-    // This is defined in the platform .cpp file
-    static GLWindowContext* Create(void* platformData, const DisplayParams& params);
-
     sk_sp<SkSurface> getBackbufferSurface() override;
 
     bool isValid() override { return SkToBool(fBackendContext.get()); }
@@ -40,11 +37,18 @@ public:
     }
 
 protected:
-    GLWindowContext(void*, const DisplayParams&);
-    void initializeContext(void*, const DisplayParams&);
-    virtual void onInitializeContext(void*, const DisplayParams&) = 0;
+    GLWindowContext(const DisplayParams&);
+    // This should be called by subclass constructor. It is also called when window/display
+    // parameters change. This will in turn call onInitializeContext().
+    void initializeContext();
+    virtual void onInitializeContext() = 0;
+
+    // This should be called by subclass destructor. It is also called when window/display
+    // parameters change prior to initializing a new GL context. This will in turn call
+    // onDestroyContext().
     void destroyContext();
     virtual void onDestroyContext() = 0;
+
     virtual void onSwapBuffers() = 0;
 
     SkAutoTUnref<const GrGLInterface> fBackendContext;