Remove no-op code in SkAlphaRuns::add
Remove no-op code in SkAlphaRuns::add

The old code says

        runs += x + 1;
        alpha += x + 1;
        x = 0;
        lastAlpha += x; // we don't want the +1

The last line does nothing. Remove this unnecessary line.

BUG=skia:401

Review URL: https://codereview.chromium.org/1506253002

rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,HalCanary/skia-hc,tmpvar/skia.cc,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,aosp-mirror/platform_external_skia,qrealka/skia-hc,qrealka/skia-hc,rubenvb/skia,rubenvb/skia,qrealka/skia-hc,tmpvar/skia.cc,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,tmpvar/skia.cc,google/skia,qrealka/skia-hc,rubenvb/skia,google/skia,qrealka/skia-hc,google/skia,tmpvar/skia.cc,tmpvar/skia.cc,google/skia,qrealka/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,qrealka/skia-hc,qrealka/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc