sk_sp: check for self-assignment
sk_sp: check for self-assignment

Change-Id: I02e6ab8945c79c14458b36bd13ce6f6e14979460
Reviewed-on: https://skia-review.googlesource.com/126683
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Auto-Submit: Hal Canary <fd9641b23487c8ee78406c27513624c460600766@google.com>
Commit-Queue: Hal Canary <fd9641b23487c8ee78406c27513624c460600766@google.com>

google/skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia