DevTools: Remove unnecessary tabindex on sources panel editorview

Gif: https://imgur.com/a/eP7Irdr

Bug: 963183
Change-Id: I8dd5a9a5abc4d1404385b924cbfe110e4bb62132
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804522Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Anubha Mathur <anubmat@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#697359}

third_party/blink/renderer/devtools/front_end/sources/SourcesPanel.js

@@ -65,7 +65,6 @@ Sources.SourcesPanel = class extends UI.Panel {
     const initialNavigatorWidth = 225;
     this.editorView = new UI.SplitWidget(true, false, 'sourcesPanelNavigatorSplitViewState', initialNavigatorWidth);
     this.editorView.enableShowModeSaving();
-    this.editorView.element.tabIndex = 0;
     this._splitWidget.setMainWidget(this.editorView);
 
     // Create navigator tabbed pane with toolbar.