This CL is targeted to improve the objective/subjective quality of the
"ext-refs" coding tool.

Tuned the frame rate factors as follows:

(1) BRF_UPDATE:
    Decreased from INTER_HIGH (1.5) to GF_ARF_LOW (1.25);
(2) Both LAST_BIPRED_UPDATE and BIPRED_UPDATE:
    Increased from INTER_LOW (0.80) to INTER_NORMAL (1.00)
, which is to reduce the bits allocated to the BWEDREF frame whereas
to increase the bits allocated to the bi-directionally predicted
frames.

Obtained a coding gain in overall PSNR as follows, compared against
the original ext-refs:

lowres: BDRate -0.181%
midres: BDRate -0.090%
hdres:  BDRate -0.701%

Change-Id: Id001f3b124d833da9f80eba4cdec2ca3f6ba9d82