Switch the default 2 pass vbr variant.
Switch the default 2 pass vbr variant.

On balance Deb's  modified rate control for VBR seems
to be outperforming especially on some low motion YT
clips so I have switched this to be the default mode for
now.

Change-Id: I0713d430cad6425ac5c48fccdf332e12814ee44a

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