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Previously, an oscillator would start at the nearest sample frame
boundary.  This can produce noticeable effects.

Instead, make the oscillator start at the requested sample time and
sample the curve appropriately.

BUG=631576
TEST=Oscillator/start-sampling.html

Review-Url: https://codereview.chromium.org/2186813003
Cr-Original-Commit-Position: refs/heads/master@{#442669}
Committed: https://chromium.googlesource.com/chromium/src/+/7909df464ca46a1ea121b0c54e5d04cfa521e38c
Review-Url: https://codereview.chromium.org/2186813003
Cr-Commit-Position: refs/heads/master@{#443040}