fix more 64bit warnings

BUG=skia:

Review URL: https://codereview.chromium.org/147683003

git-svn-id: http://skia.googlecode.com/svn/trunk@13190 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 3b953f4..287639a 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -128,7 +128,7 @@
         return false;
     }
 
-    int len = stream.getLength();
+    size_t len = stream.getLength();
     SkString data(len);
     stream.read(data.writable_str(), len);
     const char* s = data.c_str();