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}
void ExtensionInstallPrompt::OnInstallFailure(
    const extensions::CrxInstallError& error) {
  install_ui_->OnInstallFailure(error);
}

void ExtensionInstallPrompt::OnInstallFailure(
    const extensions::CrxInstallError& error) {
  install_ui_->OnInstallFailure(error);
}
