Inline performance-critical methods on SkAlphaRuns.

Clang needs to be forced to inline add, so we do that.

GCC on desktop
              bench conf time        old        new       diff    diffP
draw_coverage_false 8888 c        236.08     212.01     +24.07   +10.2%
 draw_coverage_true 8888 c        125.67     104.49     +21.18   +16.9%

GCC on N5
              bench conf time        old        new       diff    diffP
draw_coverage_false 8888 c        566.33     529.96     +36.37    +6.4%
 draw_coverage_true 8888 c        315.39     293.85     +21.54    +6.8%

Clang on desktop
              bench conf time        old        new       diff    diffP
draw_coverage_false 8888 c        337.37     330.63      +6.74    +2.0%
 draw_coverage_true 8888 c        126.87     119.32      +7.55    +6.0%

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/83243005

git-svn-id: http://skia.googlecode.com/svn/trunk@12370 2bbb7eff-a529-9590-31e7-b0007b416f81