Revert of Do not interchange MOUSE_MOVED and MOUSE_DRAGGED events in Widget (patchset...

Revert of Do not interchange MOUSE_MOVED and MOUSE_DRAGGED events in Widget (patchset #8 id:140001 of https://codereview.chromium.org/560053002/)

Reason for revert:
Responsible for the crash reported in crbug.com/430427. Relanding will require addressing crbug.com/431350 first.

Original issue's description:
> Do not interchange MOUSE_MOVED and MOUSE_DRAGGED events in Widget
>
> In Widget::OnMouseEvent(), do not handle MOUSE_MOVED
> events as MOUSE_DRAGGED events, and do not handle
> MOUSE_DRAGGED events as MOUSE_MOVED events; instead
> trust that the mouse event type has been set correctly
> before arriving at Widget. This also allows us to
> remove the member |is_mouse_button_pressed_|
> from Widget.
>
> BUG=412929, 412931
> TEST=WidgetTest.DragIntoView

BUG=412929, 412931
TBR=sadrul@chromium.org

Review URL: https://codereview.chromium.org/703643004

Cr-Commit-Position: refs/heads/master@{#303274}