Refactor partial screenshot region selector (2nd)

Refactor partial screenshot region selector (2nd)

This is the reland of crrev.com/308508 which was reverted due
to a failure of valgrind bot.

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 with valgrind bot

Review URL: https://codereview.chromium.org/813523002

Cr-Commit-Position: refs/heads/master@{#308942}