Disallow non-postive sizes in vector icon views example.

This is mostly a follow-up to an unintentional behavior change in
https://codereview.chromium.org/2255403002/ .  I decided to treat 0 size as
invalid as well.

BUG=none
TEST=none

Review-Url: https://codereview.chromium.org/2278743002
Cr-Commit-Position: refs/heads/master@{#414215}

ui/views/examples/vector_example.cc

@@ -97,7 +97,7 @@ class VectorIconGallery : public View,
   void ContentsChanged(Textfield* sender,
                        const base::string16& new_contents) override {
     if (sender == size_input_) {
-      if (base::StringToInt(new_contents, &size_))
+      if (base::StringToInt(new_contents, &size_) && (size_ > 0))
         UpdateImage();
       else
         size_input_->SetText(base::string16());