Bug 1553451 - Lower the eslint complexity level to 20 for devtools and exlude more complex functions; r=jdescottes

We want to go back to ESLint's default complexity level so that newly introduced code is checked for complexity.
At the same time, to make that work, we're excluding all of the more complex functions for now.
We should fix them: make them less complex, and remove the eslint-disable comment.
See bug 1553449 for more information about this.

Differential Revision: https://phabricator.services.mozilla.com/D32139

devtools/client/shared/components/tree/TreeView.js

@@ -270,6 +270,7 @@ define(function(require, exports, module) {
 
     // Event Handlers
 
+    /* eslint-disable complexity */
     onKeyDown(event) {
       if (!SUPPORTED_KEYS.includes(event.key)) {
         return;
@@ -352,6 +353,7 @@ define(function(require, exports, module) {
       this.treeRef.current.focus();
       event.preventDefault();
     }
+    /* eslint-enable complexity */
 
     onClickRow(nodePath, event) {
       const onClickRow = this.props.onClickRow;