fix warning from scalar --> int32 conversion

BUG=

Review URL: https://codereview.appspot.com/7065050

git-svn-id: http://skia.googlecode.com/svn/trunk@7061 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrMemoryPool.cpp b/src/gpu/GrMemoryPool.cpp
index 17d70ab..2518a74 100644
--- a/src/gpu/GrMemoryPool.cpp
+++ b/src/gpu/GrMemoryPool.cpp
@@ -119,6 +119,7 @@
 }
 
 void GrMemoryPool::validate() {
+#ifdef SK_DEBUG
     BlockHeader* block = fHead;
     BlockHeader* prev = NULL;
     GrAssert(block);
@@ -156,5 +157,6 @@
     } while ((block = block->fNext));
     GrAssert(allocCount == fAllocationCnt);
     GrAssert(prev == fTail);
+#endif
 }