Make SkEmptyPicture a singleton.
Make SkEmptyPicture a singleton.

In my confusion yesterday I accidentally left this as a non-singleton.
The issue in Blink was not related to this being a singleton,
and it should be safe to make it one.

This means recording an empty picture properly costs zero mallocs.

BUG=skia:

Review URL: https://codereview.chromium.org/1147053002

vanish87/skia,qrealka/skia-hc,shahrzadmn/skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,ominux/skia,HalCanary/skia-hc,shahrzadmn/skia,Jichao/skia,qrealka/skia-hc,Jichao/skia,ominux/skia,tmpvar/skia.cc,nvoron23/skia,todotodoo/skia,shahrzadmn/skia,vanish87/skia,nvoron23/skia,google/skia,rubenvb/skia,ominux/skia,vanish87/skia,google/skia,HalCanary/skia-hc,pcwalton/skia,HalCanary/skia-hc,pcwalton/skia,nvoron23/skia,noselhq/skia,google/skia,pcwalton/skia,HalCanary/skia-hc,ominux/skia,tmpvar/skia.cc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Jichao/skia,shahrzadmn/skia,todotodoo/skia,rubenvb/skia,shahrzadmn/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,HalCanary/skia-hc,qrealka/skia-hc,HalCanary/skia-hc,tmpvar/skia.cc,rubenvb/skia,Jichao/skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,nvoron23/skia,aosp-mirror/platform_external_skia,pcwalton/skia,vanish87/skia,Jichao/skia,pcwalton/skia,noselhq/skia,noselhq/skia,vanish87/skia,aosp-mirror/platform_external_skia,Jichao/skia,noselhq/skia,rubenvb/skia,shahrzadmn/skia,ominux/skia,nvoron23/skia,vanish87/skia,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,ominux/skia,aosp-mirror/platform_external_skia,Jichao/skia,rubenvb/skia,noselhq/skia,todotodoo/skia,ominux/skia,rubenvb/skia,todotodoo/skia,nvoron23/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,noselhq/skia,vanish87/skia,nvoron23/skia,Hikari-no-Tenshi/android_external_skia,tmpvar/skia.cc,noselhq/skia,pcwalton/skia,ominux/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,noselhq/skia,qrealka/skia-hc,todotodoo/skia,pcwalton/skia,todotodoo/skia,pcwalton/skia,google/skia,HalCanary/skia-hc,google/skia,todotodoo/skia,Jichao/skia,rubenvb/skia,pcwalton/skia,tmpvar/skia.cc,qrealka/skia-hc,qrealka/skia-hc,todotodoo/skia,ominux/skia,vanish87/skia,shahrzadmn/skia,qrealka/skia-hc,Jichao/skia,nvoron23/skia,nvoron23/skia,aosp-mirror/platform_external_skia,qrealka/skia-hc,shahrzadmn/skia,vanish87/skia,todotodoo/skia,tmpvar/skia.cc,google/skia,noselhq/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,shahrzadmn/skia