Move MediaStream and MediaStreamTrack implementation from modules/mediastream to core/mediastream.

LayoutTests/fast/mediastream/RTCPeerConnection-stats-expected.txt

-Tests the RTCPeerConnection stats interface.
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-On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
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-PASS pc = new webkitRTCPeerConnection(null) did not throw exception.
-PASS pc.getStats(statsHandler1) did not throw exception.
-PASS statsHandler1 was called
-PASS status is non-null.
-PASS result.length is 0
-PASS getUserMedia({audio:true, video:true}, gotStream) did not throw exception.
-PASS Got a stream.
-PASS pc.getStats(statsHandler2) did not throw exception.
-PASS statsHandler2 was called
-PASS result.length is >= 2
-PASS timediff is >= 0
-PASS local.names().length is >= 1
-PASS local.names().indexOf("type") is >= 0
-PASS local.stat("type") is "audio"
-PASS timediff is >= 0
-PASS res.id is non-null.
-PASS res.type is non-null.
-PASS res.names().length is >= 1
-PASS res.names().indexOf("type") is >= 0
-PASS res.stat("type") is "audio"
-PASS status_g.namedItem(res.id) is non-null.
-PASS status_g[res.id] is non-null.
-PASS pc.close() did not throw exception.
-PASS pc.getStats(statsHandler3) did not throw exception.
-PASS statsHandler3 was called
-PASS result.length is >= 2
-PASS timediff is >= 0
-PASS local.names().length is >= 1
-PASS local.names().indexOf("type") is >= 0
-PASS local.stat("type") is "audio"
-PASS timediff is >= 0
-PASS res.id is non-null.
-PASS res.type is non-null.
-PASS res.names().length is >= 1
-PASS res.names().indexOf("type") is >= 0
-PASS res.stat("type") is "audio"
-PASS status_g.namedItem(res.id) is non-null.
-PASS status_g[res.id] is non-null.
-PASS successfullyParsed is true
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-TEST COMPLETE
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