Update test threshold for Mac 10.11

The threshold for the -100 Hz sine wave is apparently just a little
too tight for MacOS 10.11.  Adjust the threshold slightly.

Bug: 1104535
Change-Id: I7102093e2641a748d07ca63c05e9e8e63c9f8dce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2295211Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787847}

third_party/blink/web_tests/external/wpt/webaudio/the-audio-api/the-oscillatornode-interface/osc-basic-waveform.html

@@ -76,7 +76,7 @@
               a1: 0,
               b1: 1,
               prefix: 'Sine',
-              threshold: 4.1724e-7,
+              threshold: 4.7684e-7,
               snrThreshold: 130.95
             });
             task.done();