How long until we see the first infoleak bug where some combination of OS+driver+browser+webgl-command-sequence misses a buffer to clear - or "optimizes" it away - or fails to bounds-check a texture coordinate - etc? :)
You don't need webgl for this kind of infoleak either, regular good old 2d canvas also supports allocating memory. It also supports reading the current state of all of the pixels in the buffer through Javascript, so if you have an exploit that gets you an uninitialized canvas you can easily send whatever memory contents you got back to your server for later analysis.
If the DOM element you draw has the same origin as your canvas it seems like (from my reading of the spec) you should be allowed to do what you describe.
WebGL being enabled by default is insanity in my opinion.