Fix ParseElementHeader to support 0 payload elements

Cherry-pick'ing Change 5c83bbec9a5f6f00a349674ddad85b753d2ea219
from upstream. This fixes regression in some edge cases for mkv
playback.

BUG=26499283

Change-Id: I88de03219a3d941b6b2f251d384e29c36bdd4d9b

long long VideoTrack::GetDisplayUnit() const { return m_display_unit; }

long long VideoTrack::GetDisplayUnit() const { return m_display_unit; }
