D3D11 interprets an index value of 0xFFFF in a 16-bit index buffer as a
primitive restart marker. This behavior can't be toggled off.
http://msdn.microsoft.com/en-us/library/windows/desktop/bb205124(v=vs.85).aspx
We work around it by converting to 32-bit indices.

BUG=angle:708

Change-Id: Ibc92d6ba98e5f11a98d76cae14f90ca050a19964