According to
msdn.microsoft.com/en-us/library/windows/desktop/bb173075(v=vs.85).aspx
the front buffer is the desktop and is not a dedicated part of the swap chain.
Thus we should only need 1 swap chain buffer instead of the currently allocated
2. Samples in the DirectX SDK also only allocate just one.

BUG=angle:662

Change-Id: If88eeab1954cbc92b39ac7cf42f924515c4b77f9
Reviewed-on: https://chromium-review.googlesource.com/201860Tested-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>