remove (unnecessary?) call to notifyPixelsChanged

BUG=skia:

Review URL: https://codereview.chromium.org/1265733002
diff --git a/src/core/SkPixelRef.cpp b/src/core/SkPixelRef.cpp
index 56bcc9a..8e0f212 100644
--- a/src/core/SkPixelRef.cpp
+++ b/src/core/SkPixelRef.cpp
@@ -365,7 +365,6 @@
 void SkPixelRef::restoreMutability() {
     SkASSERT(fMutability != kImmutable);
     fMutability = kMutable;
-    this->notifyPixelsChanged();  // This is just precautionary.
 }
 
 bool SkPixelRef::readPixels(SkBitmap* dst, const SkIRect* subset) {