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/animator/SkDisplayPost.cpp b/src/animator/SkDisplayPost.cpp
index a50d5cc..21cc9a7 100644
--- a/src/animator/SkDisplayPost.cpp
+++ b/src/animator/SkDisplayPost.cpp
@@ -235,7 +235,7 @@
     const char* ch = sink.c_str();
     do {
         const char* end = strchr(ch, '.');
-        size_t len = end ? end - ch : strlen(ch);
+        size_t len = end ? (size_t) (end - ch) : strlen(ch);
         SkDisplayable* displayable = NULL;
         if (SK_LITERAL_STR_EQUAL("parent", ch, len)) {
             if (fTargetMaker->fParentMaker)