External SkImageGenerator API

Introduce an SkImageGenerator API to support the implementation of
externally-managed image decode and scale caches.

BUG=skia:5806
R=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4720

Change-Id: Ibfe37af5471f78f28f88f9d5e80938882be1a344
Reviewed-on: https://skia-review.googlesource.com/4720
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/gn/gm.gni b/gn/gm.gni
index 0b11b2e..7e70c89 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -160,6 +160,7 @@
   "$_gm/imagefiltersstroked.cpp",
   "$_gm/imagefilterstransformed.cpp",
   "$_gm/imagefromyuvtextures.cpp",
+  "$_gm/imagegeneratorexternal.cpp",
   "$_gm/imagemagnifier.cpp",
   "$_gm/imagemakewithfilter.cpp",
   "$_gm/imagemasksubset.cpp",