Set tooltip only when actually hovering on the battery view.

This is a follow-up of crrev.com/c/1306969. After that change it showed
the tooltip for power status even when hovering on other parts of the
tray.

Bug: 888072
Change-Id: I26c5786a8ff19347f2eb58dd1430e2d5ea66d066
Reviewed-on: https://chromium-review.googlesource.com/c/1308961Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604204}

ash/system/power/tray_power.cc

@@ -58,7 +58,7 @@ void PowerTrayView::GetAccessibleNodeData(ui::AXNodeData* node_data) {
 }
 
 views::View* PowerTrayView::GetTooltipHandlerForPoint(const gfx::Point& point) {
-  return this;
+  return GetLocalBounds().Contains(point) ? this : nullptr;
 }
 
 bool PowerTrayView::GetTooltipText(const gfx::Point& p,