Under-the-hood improvements to Device Art Generator.

Under-the-hood improvements to Device Art Generator.

Use Canvas.toBlob with a polyfill, to avoid img src with massive data URLs, which causes some browsers to crash

Also change the resulting filename to be more appropriate, and make dragging-out to your desktop use this filename.

Change-Id: I13d8b4e119af8fecf567ecd6ccf3bb3e7b2d0fae
(cherry picked from commit 9da49f160612433e8ba562796ca5d6486ab48d83)