Enable context menu at drag-end for webui tabstrip.

Bug: 1096189
Change-Id: Ie6d6bb110580d5b96751331483bac6ac46153f0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367991Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806370}

chrome/browser/ui/webui/tab_strip/chrome_content_browser_client_tab_strip_part.cc

@@ -37,4 +37,5 @@ void ChromeContentBrowserClientTabStripPart::OverrideWebkitPrefs(
   web_prefs->minimum_logical_font_size =
       default_prefs.minimum_logical_font_size;
   web_prefs->touch_drag_drop_enabled = true;
+  web_prefs->touch_dragend_context_menu = true;
 }