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This adds three tests: before metadata, after metadata, and after
have enough. It doesn't clone the have future data test that src=
has since has_future_data == has_enough_data when using MSE.

BUG=756897
TEST=all tests!

Change-Id: I99c2cff3668c471bcd3d3d4549e53690788b0813
Reviewed-on: https://chromium-review.googlesource.com/989073Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548575}