ack-buffer contains a keyframe prior to the seek position.

 <p>Note: Implementations should return a single value. Dynamic changes to the back-buffer are
 not currently supported.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>The duration of media to retain in the buffer prior to the current playback position,
     in microseconds.</dd>
</dl>
</li>
</ul>
<a id="retainBackBufferFromKeyframe()">
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<h4>retainBackBufferFromKeyframe