DevTools: Fix focus when selecting to the left of treeitems

The mousedown listener in ElementsTreeOutline was moving focus, but
not consuming the event. The event then propagated to the treeoutline
element itself, which stole focus.

Change-Id: I2de2001175f4029cd94880297c460ff4c90b845a
Reviewed-on: https://chromium-review.googlesource.com/c/1387688Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618423}

third_party/blink/renderer/devtools/front_end/elements/ElementsTreeOutline.js

@@ -560,6 +560,7 @@ Elements.ElementsTreeOutline = class extends UI.TreeOutline {
       return;
 
     element.select();
+    event.consume(true);
   }
 
   /**