Don't clip FFT output values to minDecibels.

If the (linear) FFT value is zero, the dB value was set to
analyser.minDecibels.  The spec doesn't require this and it makes more
sense to return the expected -Infinity instead.

BUG=588853
TEST=realtimeanalyser-zero.html

Review URL: https://codereview.chromium.org/1729783002

Cr-Commit-Position: refs/heads/master@{#378081}
4 files changed