ng: add plugin-id on tab for testability (tensorflow#3591)

We would like our webdriver tests to be able to find these nodes without
inspecting the textNode (with plugin name as opposed to plugin id). This
change adds data attribute adds one.

tensorboard/webapp/header/plugin_selector_component.ng.html

@@ -27,6 +27,7 @@
       <span
         class="plugin-name"
         (click)="onActivePluginSelection($event, plugin.id)"
+        [attr.data-plugin-id]="plugin.id"
       >
         {{plugin.tab_name}}
       </span>
@@ -39,7 +40,11 @@
     [value]="selectedPlugin"
     (selectionChange)="onDisabledPluginSelectionChanged($event)"
   >
-    <mat-option *ngFor="let plugin of disabledPlugins" [value]="plugin.id">
+    <mat-option
+      *ngFor="let plugin of disabledPlugins"
+      [value]="plugin.id"
+      [attr.data-plugin-id]="plugin.id"
+    >
       {{ plugin.tab_name }}
     </mat-option>
   </mat-select>