Devtools: change "more tabs" dropdown button focus color

Issue: the "more tabs" dropdown button doesn't have enough contrast when
hover/keyboard focused in dark theme

This change changes the hover/focused color to match the rest tool bar button

Screenshots: https://imgur.com/a/TBoOlzR

Bug: 963183

Change-Id: I9ff66c3be732cf598d18d71af643d01a7de5ce54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841580Reviewed-by: Lorne Mitchell <lomitch@microsoft.com>
Commit-Queue: Junyi Xiao <juxiao@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#704080}

third_party/blink/renderer/devtools/front_end/ui/tabbedPane.css

@@ -182,8 +182,7 @@
 
 .tabbed-pane-header-tabs-drop-down-container:hover,
 .tabbed-pane-header-tabs-drop-down-container[data-keyboard-focus="true"]:focus {
-    background-color: rgb(229, 229, 229);
-    opacity: 1.0;
+    background-color: rgba(0, 0, 0, 0.08);
 }
 
 .tabbed-pane-header-tabs-drop-down-container.measuring {