tell our grcontext to abandon its backend

BUG=skia:

Review URL: https://codereview.chromium.org/692803003

samplecode/SampleApp.cpp

@@ -259,8 +259,12 @@ public:
 
     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;

src/views/mac/SkNSView.mm

@@ -386,6 +386,7 @@ CGLContextObj createGLContext(int msaaSampleCount) {
         andGetInfo:(SkOSWindow::AttachmentInfo*) info {
     if (nil == fGLContext) {
         CGLContextObj ctx = createGLContext(sampleCount);
+        SkASSERT(ctx);
         fGLContext = [[NSOpenGLContext alloc] initWithCGLContextObj:ctx];
         CGLReleaseContext(ctx);
         if (NULL == fGLContext) {