Speedup VP8LGetHistoImageSymbols for low effort (m=0) mode.

- Do light weight entropy based histogram combine and leave out CPU
  intensive stochastic and greedy heuristics for combining the
  histograms.

For 1000 image PNG corpus (m=0), this change yields speedup of 10% at
lower quality range (1% drop in compression density) and about 5% for
higher quality range (1% drop in compression density). Following is the
compression stats (before/after) for method = 0:
         Before           After
         bpp/MPs          bpp/MPs
q=0      2.8336/16.577    2.8615/18.000
q=5      2.8336/16.504    2.8615/18.216
q=10     2.8293/16.419    2.8572/18.070
q=15     2.8242/17.582    2.8519/18.371
q=20     2.8182/16.131    2.8454/18.975
q=25     2.7924/7.670     2.8230/8.531
q=30     2.7078/6.635     2.7310/7.706
q=35     2.7028/6.203     2.7253/6.855
q=40     2.7005/6.198     2.7231/6.364
q=45     2.6989/5.570     2.7216/5.844
q=50     2.6970/5.087     2.7196/5.210
q=55     2.6963/4.589     2.7208/4.766
q=60     2.6949/4.292     2.7195/4.495
q=65     2.6940/3.970     2.7185/4.024
q=70     2.6929/3.698     2.7174/3.699
q=75     2.6919/3.427     2.7164/3.449
q=80     2.6918/3.106     2.7161/3.222
q=85     2.6909/2.856     2.7153/2.919
q=90     2.6902/2.695     2.7145/2.766
q=95     2.6881/2.499     2.7124/2.548
q=100    2.6873/2.253     2.6873/2.285

Change-Id: I0567945068f8dc7888041e93d872f9def91f50ba