Fix SkOneShotDiscardablePixelRef to return discardable segment.
Fix SkOneShotDiscardablePixelRef to return discardable segment.

The SkOneShotDiscardablePixelRef was not overriding
diagnostic_only_getDiscardable method of SkPixelRef. This caused the
memory to be acocunted under malloc instead of discardable and caused
wrong accounting.

BUG=528295

Review URL: https://codereview.chromium.org/1304083006

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