Fix CrOS SampleApp

Add compile-time check for GPU build when using GPU flag

TBR=robertphillips@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1376423004
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index c891508..9be7567 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -800,10 +800,12 @@
 #endif
 
     fDeviceType = kRaster_DeviceType;
+#if SK_SUPPORT_GPU
     if (FLAGS_gpu) {
         fDeviceType = kGPU_DeviceType;
     }
-
+#endif
+        
 #if DEFAULT_TO_GPU
     fDeviceType = kGPU_DeviceType;
 #endif