partial memory leak in pdfjs

```
There is a memory leak when running pdfjs for extended periods of time. The 
problem is that the "canvas_logs" array keeps growing with each run. It should 
probably be reset in the tearDownPdfJS function.
```
Original issue reported on code.google.com by hann...@gmail.com on 8 Oct 2014 at 1:16: https://github.com/chromium/octane/issues/26