Revert "DevTools: Fix focus when selecting to the left of treeitems"

This reverts commit 9a60a25d.

Reason for revert: This broke a lot of focus related logic
in the Elements panel

Bug: 920518

Original change's description:
> 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/1387688
> Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
> Commit-Queue: Joel Einbinder <einbinder@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#618423}

TBR=pfeldman@chromium.org,einbinder@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I1407655b9a6ca03a2f110f87b7b9d2f98795ef53
Reviewed-on: https://chromium-review.googlesource.com/c/1450512Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628528}

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

@@ -563,7 +563,6 @@ Elements.ElementsTreeOutline = class extends UI.TreeOutline {
       return;
 
     element.select();
-    event.consume(true);
   }
 
   /**