Refactor partial screenshot region selector.

Refactor partial screenshot region selector.

Previously PartialScreenshotView is a view inside of a frameless
window, which is actually problematic because it needs to deal
with several window manager concepts such like mouse captures
and activations.

This is now built with ui::Layer and EventHandler, therefore
it makes no effects.

Also this CL moves the file location to a new directory
ash/utility, because this is nothing related to the window manager.

BUG=330348
R=oshima@chromium.org
TEST=the new test covers.

Review URL: https://codereview.chromium.org/805793004

Cr-Commit-Position: refs/heads/master@{#308508}