Use special case for 0x00 and 0xFF alpha to go faster.

Use special case for 0x00 and 0xFF alpha to go faster.
Base          Exp           Ratio           Name
3916010   3077544   0.785888  top25desk_amazon_com.skp_1
3462512   2776580   0.801898  top25desk_google_com_calendar_.skp_1
3446330   3134187   0.909427  top25desk_ebay_com.skp_1
590474    546375    0.925316  top25desk_techcrunch_com.skp_1
3804991   3544162   0.931451  top25desk_google_com__hl_en_q_b.skp_1
996037    939960    0.9437    top25desk_blogger.skp_1
973264    922677    0.948023  top25desk_wikipedia__1_tab_.skp_1
4514050   4295660   0.95162   top25desk_docs___1_open_documen.skp_1
4255383   4110057   0.965849  top25desk_linkedin.skp_1
8408717   8191843   0.974208  top25desk_booking_com.skp_1
2878529   2806501   0.974977  top25desk_plus_google_com_11003.skp_1
11509894  11254486  0.97781   top25desk_pinterest.skp_1
9132514   9010635   0.986654  top25desk_weather_com.skp_1
6504720   6419592   0.986913  top25desk_sports_yahoo_com_.skp_1
9136774   9033870   0.988737  top25desk_answers_yahoo_com.skp_1
4836199   4799784   0.99247   top25desk_news_yahoo_com.skp_1
1393650   1384065   0.993122  top25desk_games_yahoo_com.skp_1
6779678   6735278   0.993451  top25desk_youtube_com.skp_1
10926943  10882308  0.995915  top25desk_espn.skp_1
4259514   4245489   0.996707  top25desk_facebook.skp_1
10955293  10947657  0.999303  top25desk_google_com_search_q_c.skp_1
9153575   9207386   1.00588   top25desk_twitter.skp_1
3865942   3906345   1.01045   top25desk_wordpress.skp_1
4180009   4305530   1.03003   top25desk_mail_google_com_mail_.skp_1

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2020463003

Review-Url: https://codereview.chromium.org/2020463003