taskmanager: Show highlight before ToolTipInstance starts loading for grouped tooltips

For grouped tooltips, if there are many windows in a group, it could
take a long time before all ToolTipInstances are loaded.

Show highlight before ToolTipInstance starts loading to make user feel
less unresponsive.

Note: Will not work when a tooltip is already visible (move directly
from a task to a grouped task)

CCBUG: 433907


(cherry picked from commit 818b1b29)