Fix release build and clean up externs.

The release build was drawing blurrily because the imageSmoothingEnabled
property on a CanvasRenderingContext2D isn't specified by Closure's
externs definitions.

This CL adds the missing property to our internal externs file and removes
the unnecessary RenderingContext definitions in externs.js.

R=jackhou@chromium.org

Review URL: https://codereview.appspot.com/55830043