Bug 1016413 - Implement reporting metadata with fmp4 backend. r=rillian

There are 3 files modified in order to have the reporting work.

MP4Reader::ReadMetadata() has now its aTags parameter affected with
a certain MetadataTags structure which comes from MP4Demuxer::GetTags().

MP4Demuxer::GetTags() is in the mp4_demuxer's namespace, where a
structure mapping tags key to its char* value is set.

MP4Demuxer::GetTags() loops all along this mapping structure, and
checks for each key if there is metadata corresponding to it in the
current file's extracted metadata. If there is, it stores the current
key and its corresponding metadata value into a mozilla::MetadataTags
structure.
Bug 1016413 - Tests for fmp4 metadata backend. r=rillian

The reporting of metadata with fmp4 backend is tested here with the
file gizmo.metadata.mp4, and some changes are made on test_metadata.html
to integrate tests on mp4 files with restriction to certain platforms.
A 'type' field has also been added to gMetadataTests' elements in
manifest.js in order to correctly test the type with canPlayType method
for instance.