Add GM for SkPixelXorXfermode
Add GM for SkPixelXorXfermode

This is in preparation for adding a gpu implementation

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1601743009

Review URL: https://codereview.chromium.org/1601743009

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