Adjust end to end psnr value

A recent change leads to slight difference of encoding results:
d3aaac367 Change calculation of rd multiplier,
which is caught by Jenkins nightly test.

Adjust the threshold to silence the test failure.

BUG=webm:1725

Change-Id: I7e8b3a26b72c831ae4d88d0fca681b354314739d
1 file changed