Scale 3x3 block instead of 16x16 block in each loop.

Benefits:
1. Reduced number of different phase_scaler from 16 to 3. Optimization code
   will be smaller and faster.
2. The maximum phase_scaler drifting will be reduced from 5/16 to 1/24.
   (The drifting is 1/(3*16) in each step.)

BUG=webm:1419

Change-Id: Ibb9242a629ddb03e1ff93b859bece738255e698c