Bump up allowed maxAllowedError in webaudio tests.

This allows the test to pass on linux32 after the clang switch.
The test passed on 32bit mac already with the same compiler – this turned
out to be because clang is allowed to treat sqrtf() as a builtin on mac, but not
on linux (due to glibc's math routines setting errno). See the bug for details.

BUG=392764

Review URL: https://codereview.chromium.org/426633005

git-svn-id: svn://svn.chromium.org/blink/trunk@179182 bbb929c8-8fbe-4397-9dbb-9b2b20218538