Custom buttons should only handle accelerators when focused.

BUG=541415

Review URL: https://codereview.chromium.org/1437523005

Cr-Commit-Position: refs/heads/master@{#360130}
const CustomButton* CustomButton::AsCustomButton(const views::View* view) {
  return AsCustomButton(const_cast<views::View*>(view));
}

const CustomButton* CustomButton::AsCustomButton(const views::View* view) {
  return AsCustomButton(const_cast<views::View*>(view));
}
