[turbolizer] Make the '/' shortcut work in all of multiview

Before this change, the activeElement used to be the body and not
multiview. Then, the EventListener wasn't triggering.

Bug: v8:7327
Change-Id: I9782159ffd510c9a7afd83695f20ede9e774ac20
Notry: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1868624
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64422}

tools/turbolizer/src/graphmultiview.ts

@@ -33,6 +33,7 @@ export class GraphMultiView extends View {
   createViewElement() {
     const pane = document.createElement("div");
     pane.setAttribute("id", multiviewID);
+    pane.setAttribute("tabindex", "0");
     pane.className = "viewpane";
     return pane;
   }