Update ToolbarButton's icon by itself

Previously, ToolbarButtons' icons were updated by ToolbarView.
But ToolbarButton can handle it by itself.

From now on, ToolbarButton will call UpdateIcon() ,which is a virtual
method for children classes to decide their icons, when it's needed.
e.g. When it's added to widget, theme was changed, or touch ui
      mode was changed.

TODO: we might be able to do similar things for other children views
and remove ToolbarView::Loadimages().

Bug: 1048901
Change-Id: Ia8403a48d3b7efb94df83738d113ee06c31f1258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2253600
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788928}