Add buggy GPU baselines for MultiPictureDraw Sierpinski GM (with bug label)

I am checking in these baseline images even though they are afflicted by a bug (skia:2979).

NOTRY=true
NOTREECHECKS=true
BUG=skia:2979
TBR=jvanverth@google.com

Review URL: https://codereview.chromium.org/619873003
diff --git a/expectations/gm/Test-Win7-ShuttleA-HD2000-x86-Release/expected-results.json b/expectations/gm/Test-Win7-ShuttleA-HD2000-x86-Release/expected-results.json
index 14988e7..8929f98 100644
--- a/expectations/gm/Test-Win7-ShuttleA-HD2000-x86-Release/expected-results.json
+++ b/expectations/gm/Test-Win7-ShuttleA-HD2000-x86-Release/expected-results.json
@@ -9328,6 +9328,18 @@
       ], 
       "reviewed-by-human": true
     }, 
+    "multipicturedraw_sierpinski_simple_gpu.png": {
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          3433597547108111514
+        ]
+      ], 
+      "bugs": [
+        2979
+      ], 
+      "reviewed-by-human": true
+    }, 
     "multipicturedraw_sierpinski_tiled_565.png": {
       "allowed-digests": [
         [
@@ -9346,6 +9358,18 @@
       ], 
       "reviewed-by-human": true
     }, 
+    "multipicturedraw_sierpinski_tiled_gpu.png": {
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          1779439780438634908
+        ]
+      ], 
+      "bugs": [
+        2979
+      ], 
+      "reviewed-by-human": true
+    }, 
     "nested_aa_565.png": {
       "allowed-digests": [
         [