Show correct resize coordinates  and take zoom into account when shown.
Show correct resize coordinates  and take zoom into account when shown.

Use size of canvasPlaceholder rather than size of displayCanvas to size
of current canvas, when a resize operation is taking place.
Also multiply the current size by the inverse of zoom, to show correct
size at specified zoom level.

BUG=https://github.com/jackhou-chromium/bitmapper/issues/84
R=tapted@chromium.org

Review URL: https://codereview.appspot.com/196650044

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