Enable the consistent-return ESLint rule

This rule is already enabled in mozilla-central, and helps ensure more consistent functions/methods, see https://searchfox.org/mozilla-central/rev/b9da45f63cb567244933c77b2c7e827a057d3f9b/tools/lint/eslint/eslint-plugin-mozilla/lib/configs/recommended.js#119-120

Please see https://eslint.org/docs/rules/consistent-return for additional information.

src/display/canvas.js

@@ -1435,7 +1435,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
 
       var fontSize = current.fontSize;
       if (fontSize === 0) {
-        return;
+        return undefined;
       }
 
       var ctx = this.ctx;