Update to Chromium version 68.0.3409.0 (#553714)

Known issues:
- Download fails with empty cache path due to https://crrev.com/950748a25f.
  Fixed in https://crrev.com/087ef223ac.

libcef/browser/native/browser_platform_delegate_native_linux.cc

@@ -383,7 +383,8 @@ void CefBrowserPlatformDelegateNativeLinux::TranslateMouseEvent(
                       TranslateModifiers(mouse_event.modifiers));
 
   // timestamp
-  result.SetTimeStampSeconds(GetSystemUptime());
+  result.SetTimeStamp(base::TimeTicks() +
+                      base::TimeDelta::FromSeconds(GetSystemUptime()));
 
   result.pointer_type = blink::WebPointerProperties::PointerType::kMouse;
 }