Revert of Remove the fall-through for mouse moves in View::OnMouseEvent() (patchset...

Revert of Remove the fall-through for mouse moves in View::OnMouseEvent() (patchset #3 id:40001 of https://codereview.chromium.org/627463002/)

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

Original issue's description:
> Remove the fall-through for mouse moves in View::OnMouseEvent()
>
> Mouse move events should not be processed as
> mouse drag events, since if a mouse button is
> down at the time of event creation then the
> event would have been created as a drag event.
> Remove the corresponding fall-through in
> View::OnMouseEvent().
>
> BUG=412929
> TEST=covered by existing tests
>
> Committed: https://crrev.com/0ac6cd424cad20804a14199341cce41f0489d187
> Cr-Commit-Position: refs/heads/master@{#300919}

TBR=sadrul@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=412929

Review URL: https://codereview.chromium.org/709443006

Cr-Commit-Position: refs/heads/master@{#303257}