DevTools: Fix 1px gap on the console group nesting level marker

The border was missing a pixel, and so didn't look like a solid line.

https://imgur.com/a/jjEPH9g

Change-Id: I647953188d2029670851201cb7b2d43e09cf37d1
Reviewed-on: https://chromium-review.googlesource.com/1244113
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594091}

third_party/blink/renderer/devtools/front_end/console/consoleView.css

@@ -271,6 +271,7 @@
     border-right: 1px solid #a5a5a5;
     position: relative;
     margin-bottom: -1px;
+    margin-top: -1px;
 }
 
 .console-message-wrapper:last-child .nesting-level-marker::before,