Manually serialize and deserialize the canvas capture to/from JSON and

[kenrussell]

split it into chunks in order to work around new message size limits
imposed in Chrome.

Fixes #69.

[kenrussell]

Note: tested by running the extension against https://webglsamples.org/aquarium/aquarium.html with the default number of fish. Crashes in Chrome Canary without this fix, works with it. Also tested a few of the other SpectorJS demos; they all work.

Also tried https://s3.amazonaws.com/mozilla-games/ZenGarden/EpicZenGarden.html , but when SpectorJS attempts to capture a frame of that demo, it still crashes the renderer process with an out-of-memory error. The new behavior's no worse than the previous behavior, though SpectorJS used to work against this demo and it would be great if we could get it working again by reducing its memory consumption.

[sebavan]

Thanks a lot !!!