Adds WebElement::imageContents()

This function returns the image contents for a given WebElement, if there is appropriate image data.
This can be useful for implementing context-menu based actions on images. Currently it only fetches
bitmap data for <img>s, but we could extend it to other image-like nodes if we wanted to.

R=darin@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/19856004

git-svn-id: svn://svn.chromium.org/blink/trunk@155179 bbb929c8-8fbe-4397-9dbb-9b2b20218538