Make the webstore inline install dialog be tab-modal

Also clean up a few minor lint errors while I'm in here.

BUG=550047

Review URL: https://codereview.chromium.org/1496033003

Cr-Commit-Position: refs/heads/master@{#363925}
int ExtensionInstallDialogView::GetDefaultDialogButton() const {
  return ui::DIALOG_BUTTON_CANCEL;
}

int ExtensionInstallDialogView::GetDefaultDialogButton() const {
  return ui::DIALOG_BUTTON_CANCEL;
}
