Bug 1019551 - Fix failure to print when pages contain zero sized <canvas>. r=lsalzman

dom/html/HTMLCanvasElement.cpp

@@ -584,17 +584,23 @@ HTMLCanvasElement::CopyInnerTo(Element* aDest,
     HTMLCanvasElement* dest = static_cast<HTMLCanvasElement*>(aDest);
     dest->mOriginalCanvas = this;
 
-    nsCOMPtr<nsISupports> cxt;
-    dest->GetContext(NS_LITERAL_STRING("2d"), getter_AddRefs(cxt));
-    RefPtr<CanvasRenderingContext2D> context2d =
-      static_cast<CanvasRenderingContext2D*>(cxt.get());
-    if (context2d && !mPrintCallback) {
-      CanvasImageSource source;
-      source.SetAsHTMLCanvasElement() = this;
-      ErrorResult err;
-      context2d->DrawImage(source,
-                           0.0, 0.0, err);
-      rv = err.StealNSResult();
+    // We make sure that the canvas is not zero sized since that would cause
+    // the DrawImage call below to return an error, which would cause printing
+    // to fail.
+    nsIntSize size = GetWidthHeight();
+    if (size.height > 0 && size.width > 0) {
+      nsCOMPtr<nsISupports> cxt;
+      dest->GetContext(NS_LITERAL_STRING("2d"), getter_AddRefs(cxt));
+      RefPtr<CanvasRenderingContext2D> context2d =
+        static_cast<CanvasRenderingContext2D*>(cxt.get());
+      if (context2d && !mPrintCallback) {
+        CanvasImageSource source;
+        source.SetAsHTMLCanvasElement() = this;
+        ErrorResult err;
+        context2d->DrawImage(source,
+                             0.0, 0.0, err);
+        rv = err.StealNSResult();
+      }
     }
   }
   return rv;