Use correct FFT scaling for WebAudio AnalyserNode

Currently, for a fixed sine-wave input, the peak FFT amplitude changes
depending on the selected FFT size. This is incorrect; the FFT amplitude
should not appreciably change when the FFT size changes.

Layout test added to verify the FFT result stays approximately constant,
independent of the FFT size used.

When this test is run using an older version of Chrome, the peak varies
from about -44 dB (FFT size 32) to -7 dB (FFT size 2048).  With this CL,
the peak is about -7 dB.  (It's not 0 dB because of the Blackman window that is applied.)

Note: this change may break existing applications that depended on this strange scaling effect.

BUG=341536

Review URL: https://codereview.chromium.org/156993002

git-svn-id: svn://svn.chromium.org/blink/trunk@166687 bbb929c8-8fbe-4397-9dbb-9b2b20218538