Bug 932820 - Workaround for ABI changes in OMX interface on HTC devices. r=doublec, a=bajaj

On some devices, HTC has extended android::OMXCodec with their own HTCOMXCodec
which contains some extension that change the ABI.

This patch merely adds padding to MediaSource::ReadOptions, as HTCOMXCodec::read
modifies an HTC-specific field in ReadOptions which doesn't exist in AOSP.