Source

File: wp-includes/customize/class-wp-customize-code-editor-control.php

public function enqueue() {
		$this->editor_settings = wp_enqueue_code_editor(
			array_merge(
				array(
					'type'       => $this->code_type,
					'codemirror' => array(
						'indentUnit' => 2,
						'tabSize'    => 2,
					),
				),
				$this->editor_settings
			)
		);
	}