Add CPU backing store for GrAtlas to reduce texture uploads

src/gpu/GrTextStrike.h

Index: src/gpu/GrTextStrike.h
diff --git a/src/gpu/GrTextStrike.h b/src/gpu/GrTextStrike.h
index 2f2b7202a2f13a10533e79818e1ef9e60a4e6b0e..cee47c5074878c10fc65aceca215b37816d8a512 100644
--- a/src/gpu/GrTextStrike.h
+++ b/src/gpu/GrTextStrike.h
@@ -90,6 +90,14 @@ public:
     }
     GrTextStrike* getHeadStrike() const { return fHead; }
 
+    void updateTextures() {
+        for (int i = 0; i < kAtlasCount; ++i) {
+            if (fAtlasMgr[i]) {
+                fAtlasMgr[i]->uploadPlotsToTexture();
+            }
+        }
+    }
+
 #ifdef SK_DEBUG
     void validate() const;
 #else