tell our grcontext to abandon its backend

BUG=skia:

Review URL: https://codereview.chromium.org/692803003
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 1b4a55e..3d2aa7c 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -259,8 +259,12 @@
 
     virtual void tearDownBackend(SampleWindow *win) {
 #if SK_SUPPORT_GPU
-        SkSafeUnref(fCurContext);
-        fCurContext = NULL;
+        if (fCurContext) {
+            // in case we have outstanding refs to this guy (lua?)
+            fCurContext->abandonContext();
+            fCurContext->unref();
+            fCurContext = NULL;
+        }
 
         SkSafeUnref(fCurIntf);
         fCurIntf = NULL;