Selection Tool.

Mouse events now hooked to canvas wrapper and hit test is used to check
when mouse events should be handled by selection canvas manager.
User selects area of canvas and this is drawn to selection source
canvas. User can drag selection and selection canvas display also
configured to zoom factor. Image then blitted to canvas on mouse down
outside selection canvas. tearDown method also added to tool interface.

R=calamity@chromium.org, jackhou@chromium.org

Review URL: https://codereview.appspot.com/61220043