Avoid regenerating cached textblobs on integer scrolls

BUG=skia:

Review URL: https://codereview.chromium.org/1062863002
diff --git a/src/gpu/GrTextBlobCache.cpp b/src/gpu/GrTextBlobCache.cpp
index 9141b17..a5ef07c 100644
--- a/src/gpu/GrTextBlobCache.cpp
+++ b/src/gpu/GrTextBlobCache.cpp
@@ -41,5 +41,10 @@
     }
     cacheBlob->fRunCount = runCount;
     cacheBlob->fPool = &fPool;
+
+#ifdef SK_DEBUG
+    cacheBlob->fTotalXError = 0;
+    cacheBlob->fTotalYError = 0;
+#endif
     return cacheBlob;
 }