fix(cdk/tree): address review comments

Author: BobobUnicorn

src/components-examples/cdk/tree/cdk-tree-nested-level-accessor/cdk-tree-nested-level-accessor-example.css

@@ -2,3 +2,8 @@
   display: flex;
   align-items: center;
 }
+
+.example-tree-node:not(.example-expandable) {
+  line-height: 40px;
+  padding-left: 40px;
+}

src/components-examples/cdk/tree/cdk-tree-nested-level-accessor/cdk-tree-nested-level-accessor-example.html

@@ -1,14 +1,13 @@
 <cdk-tree [dataSource]="dataSource" [levelAccessor]="levelAccessor">
   <!-- This is the tree node template for leaf nodes -->
   <cdk-nested-tree-node *cdkTreeNodeDef="let node" class="example-tree-node">
-    <!-- use a disabled button to provide padding for tree leaf -->
-    <button mat-icon-button disabled></button>
     {{node.name}}
   </cdk-nested-tree-node>
   <!-- This is the tree node template for expandable nodes -->
   <cdk-nested-tree-node *cdkTreeNodeDef="let node; when: hasChild"
                         [isExpandable]="node.expandable"
-                        class="example-tree-node">
+                        [style.display]="shouldRender(node) ? 'flex' : 'none'"
+                        class="example-tree-node example-expandable">
     <button mat-icon-button cdkTreeNodeToggle
             [attr.aria-label]="'Toggle ' + node.name"
             (click)="node.isExpanded = !node.isExpanded"

src/components-examples/cdk/tree/cdk-tree-nested-level-accessor/cdk-tree-nested-level-accessor-example.ts

@@ -20,6 +20,8 @@ export class CdkTreeNestedLevelAccessorExample {
   getParentNode(node: FlatFoodNode) {
     const nodeIndex = FLAT_DATA.indexOf(node);
 
+    // Determine the node's parent by finding the first preceding node that's
+    // one level shallower.
     for (let i = nodeIndex - 1; i >= 0; i--) {
       if (FLAT_DATA[i].level === node.level - 1) {
         return FLAT_DATA[i];
@@ -29,14 +31,9 @@ export class CdkTreeNestedLevelAccessorExample {
     return null;
   }
 
-  shouldRender(node: FlatFoodNode) {
-    let parent = this.getParentNode(node);
-    while (parent) {
-      if (!parent.isExpanded) {
-        return false;
-      }
-      parent = this.getParentNode(parent);
-    }
-    return true;
+  shouldRender(node: FlatFoodNode): boolean {
+    // This node should render if it is a root node or if all of its ancestors are expanded.
+    const parent = this.getParentNode(node);
+    return !parent || (!!parent.isExpanded && this.shouldRender(parent));
   }
 }