Initialize subrun variables to make Valgrind happy
Initialize subrun variables to make Valgrind happy

The check at line 1259 was failing in Valgrind, and in
the default constructor subrun.fMaskFormat is never inited.
Also set some other variables to avoid future problems.

Review URL: https://codereview.chromium.org/1315773005

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