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See screen cast in bug comment 1 for a video of what is included and not
included in this CL.

Included:
  - Dragging to select a region
  - After first drag, fine tuning via affordance circles
  - Button to capture region (can be reused for fullscreen and window
    capture)
  - Switching to and from capture region source
Not included:
  - Nudge for tablet mode (see specs)
  - Motion/animations
  - Styling for buttons
  - Drop shadow for border and affordance circles
  - Partial magnifier for affordance circle presses
  - Cursor changes

The basics of this CL is to provide a UI to select a region. The
selection has two phases, a quick selection phase to quickly grab a
region, then a fine tuning phase where users can resize/reposition the
region with some drag affordances.

For a capture region session, we add two widgets to have supporting
text and buttons, and a struct that holds data needed to draw the rest
of the UI onto the layer. The layer is updated when the selection
region has been updated from drag events.

For phase two, we use a simple formula to determine which affordance
has been pressed down on, if any. If an affordance was pressed on, we
allow resizing, but one or two points stay the same. These are stored
in the struct as anchor points, and used to compute the new selection
region when there is a drag event.

Test: manual
Bug: 1120132
Change-Id: If9c461d82306666cbfa9be7fcd521bd6caeec331
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404945Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807318}