Vp8Parser: Zero-initialize segment feature data when required
Vp8Parser: Zero-initialize segment feature data when required

When segmentation is enabled and update_segment_feature_data flag is
true, but no actual update value is provided in the stream, the value
should be assigned the default "0", instead of what was carried over
from previous frame. This applies to both quantizer and loop filter
update values.

BUG=chrome-os-partner:48886
TEST=Sample video with segment feature data
updates,crosvideo.appspot,vdatest

Review-Url: https://codereview.chromium.org/1992663002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394616}

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