DevTools: Fix 4px of white space in network big rows mode

When the network is in large rows mode, the heading is larger. The
tabbed pane when a request is selected had a margin added to match the
larger heading. But that caused a strange white space to appear above
the tabbed pane.

Change-Id: I19bdf61509b2f14c80e06113228475759da9e7d2
Reviewed-on: https://chromium-review.googlesource.com/c/1387906Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625690}

third_party/blink/renderer/devtools/front_end/network/networkPanel.css

@@ -37,7 +37,7 @@
 }
 
 .network-details-view-tall-header {
-    margin-top: 4px;
+    border-top: 4px solid var(--toolbar-bg-color);
 }
 
 .network-item-view {