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/SampleShaderText.cpp b/samplecode/SampleShaderText.cpp
index 603d67e..6855824 100644
--- a/samplecode/SampleShaderText.cpp
+++ b/samplecode/SampleShaderText.cpp
@@ -186,7 +186,7 @@
         canvas->save();
         for (size_t s = 0; s < SK_ARRAY_COUNT(shaders); s++) {
             canvas->save();
-            int i = 2*s;
+            size_t i = 2*s;
             canvas->translate(SkIntToScalar((i / testsPerCol) * colWidth),
                               SkIntToScalar((i % testsPerCol) * rowHeight));
             paint.setShader(shaders[s])->unref();