MSE: On init segment received, set need random access point flag on all track buffers

MSE: On init segment received, set need random access point flag on all track buffers

This change updates Chromium to observe the proposed text in the W3C MSE
spec per https://www.w3.org/Bugs/Public/show_bug.cgi?id=26032.
Previously, SourceBufferStream::Append() would fail and report decode
error if the first processed (and appended to SBS) frame(s) after a
non-first init segment were not random access points.
This change drops those frames by setting the 'need random access point
flag' on all track buffers true during the initialization segment
received algorithm in SourceState::OnNewConfigs().

R=acolwell@chromium.org
TEST=shiny new test that fails (on both Legacy and new FrameProcessors) without the rest of this change applied

Review URL: https://codereview.chromium.org/322273003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276914 0039d316-1c4b-4281-b951-d872f2087c98