[tools] Fix system-analyzer script panel selection

Drive-by-fix:
- fix load spinner z-index

Change-Id: I4a8f9b768ec858da4d91780ae0998a685f4438bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2624609Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72044}

tools/system-analyzer/index.mjs

@@ -248,7 +248,7 @@ class App {
 
   focusSourcePosition(sourcePosition) {
     if (!sourcePosition) return;
-    this._view.sourcePanel.focusedSourcePositions = [sourcePosition];
+    this._view.scriptPanel.focusedSourcePositions = [sourcePosition];
   }
 
   handleToolTip(event) {

tools/system-analyzer/view/log-file-reader-template.html

@@ -44,6 +44,7 @@ found in the LICENSE file. -->
   .loading #loader {
     display: block;
     position: fixed;
+    z-index: 9999;
     top: 0px;
     left: 0px;
     width: 100%;