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For low frequencies, use a 3-point or 5-point Lagrange interpolation
scheme to improve the accuracy of the oscillator instead of using
simple linear interpolation.

For a typical context, the linear interpolator is used if the
frequency >= 3.2 Hz; the 3-point, for 3.2 > frequency > 1.7 Hz; and
the 5-point for all other cases.  Thus, typical use-cases are not
impaired by this change.

Some benchmark results from running an oscillator for 1000 seconds in an
offline context.  This is the time measured from startRendering()
until the promise is delivered.  The SNR is the measured SNR between
the test signal and a reference 440 Hz signal.  The linear time was
the same before and after this CL.

linear:
time: 1844.6 ms, stddev: 154.4
SNR: 27.7 dB

3-point:
time: 2426.5 ms, stddev: 265.0
SNR: 37.2 dB

5-point:
time: 2752.6 ms, stddev: 215.1
SNR: 59.3 dB

7-point (for reference, not used now):
time: 3239.4 ms, stddev: 263.8
SNR: 82.5 dB

Bug: 773507
Test: Oscillator/osc-440hz.html
Change-Id: Ib49fab4fb27738f4f02bd41f8adcc2e3b2dd2137
Reviewed-on: https://chromium-review.googlesource.com/713576
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510133}