Notify resource caches when pixelref genID goes stale

patch from issue 954443002 at patchset 40001 (http://crrev.com/954443002#ps40001)

BUG=skia:

Review URL: https://codereview.chromium.org/950363002
diff --git a/src/core/SkBitmapCache.h b/src/core/SkBitmapCache.h
index ec8a6b8..c54f961 100644
--- a/src/core/SkBitmapCache.h
+++ b/src/core/SkBitmapCache.h
@@ -14,6 +14,10 @@
 class SkResourceCache;
 class SkMipMap;
 
+uint64_t SkMakeResourceCacheSharedIDForBitmap(uint32_t bitmapGenID);
+
+void SkNotifyBitmapGenIDIsStale(uint32_t bitmapGenID);
+
 class SkBitmapCache {
 public:
     /**