Diagnostics: Add left-panel slot to diagnostics card

Bug: 1125150
Test: browser_tests --gtest_filter=DiagnosticsApp*
Change-Id: I98f5a156a1d6a211259efacb9133c86eabb2d237
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443210
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813603}

chromeos/components/diagnostics_ui/resources/diagnostics_card.html

@@ -27,7 +27,9 @@
   <div id="title" class="title-container spacing">
     <slot name="title"></slot>
   </div>
-
+  <div>
+    <slot name="left-panel"></slot>
+  </div>
   <div id="body" class="body-container spacing">
     <slot name="body"></slot>
   </div>