Fix the mac build of the SampleApp.

My call to glViewport breaks the mac build, but benefits the Android
and linux builds.  Fix the mac build until I come up with a better
solution.

Reviewed at http://codereview.appspot.com/4628042/


git-svn-id: http://skia.googlecode.com/svn/trunk@1612 2bbb7eff-a529-9590-31e7-b0007b416f81

samplecode/SampleApp.cpp

@@ -1323,12 +1323,13 @@ void SampleWindow::onSizeChange() {
     fZoomCenterX = SkScalarHalf(this->width());
     fZoomCenterY = SkScalarHalf(this->height());
 
+#if defined(SK_BUILD_FOR_UNIX) || defined(ANDROID)
     if (fGrContext) {
         glViewport(0, 0, SkScalarRound(this->width()),
                 SkScalarRound(this->height()));
         fGrContext->resetContext();
     }
-
+#endif
     this->updateTitle();    // to refresh our config
 }