[Webaudio]: Fix reverse interpolation and wrap around (Step 2)

This is the final incremental step (2/2) for negative playbackRate
support. It updates the linear interpolation path to correctly handle
backwards direction using a virtual_start_frame bound and correctly
calculates sub-sample rendering in reverse without rounding errors.

The previous optimization rounding grain_offset to an integer frame
is now bypassed when playbackRate is not strictly 1.0, preserving
the sub-sample position for correct backward playback tests.

Because all the expected WPT failures for negative playback rates
have now been resolved, the -expected.txt baseline file is deleted.

Bug: 40719652
Change-Id: Icac5f0ab9697bb6e198ecd237b1e7e22f4edd870
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7879375
Commit-Queue: Mahesh Kannan <kmaheshb@google.com>
Reviewed-by: Michael Wilson <mjwilson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1651112}

Author: Mahesh Bharadwaj Kannan

webaudio/the-audio-api/the-audiobuffersourcenode-interface/audiobuffersource-playbackrate-dynamic-direction.html

@@ -21,7 +21,7 @@
     source.playbackRate.setValueAtTime(1.0, 128 / 44100);
     source.connect(context.destination);
 
-    source.start(0, 255);
+    source.start(0, 255 / 44100);
 
     let renderedBuffer = await context.startRendering();
     let renderedData = renderedBuffer.getChannelData(0);