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}
base::string16 ExtensionInstallPrompt::Prompt::GetPermissionsDetails(
    size_t index,
    PermissionsType permissions_type) const {
  const InstallPromptPermissions& install_permissions =
      GetPermissionsForType(permissions_type);
  CHECK_LT(index, install_permissions.details.size());
  return install_permissions.details[index];
}

base::string16 ExtensionInstallPrompt::Prompt::GetPermissionsDetails(
    size_t index,
    PermissionsType permissions_type) const {
  const InstallPromptPermissions& install_permissions =
      GetPermissionsForType(permissions_type);
  CHECK_LT(index, install_permissions.details.size());
  return install_permissions.details[index];
}
