Fix build warnings in SkAlphaThresholdFilter

BUG=None
R=bsalomon@google.com

Author: zork@chromium.org

Review URL: https://codereview.chromium.org/101763010

git-svn-id: http://skia.googlecode.com/svn/trunk@12937 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/imagealphathreshold.cpp b/gm/imagealphathreshold.cpp
index 2839744..7597090 100644
--- a/gm/imagealphathreshold.cpp
+++ b/gm/imagealphathreshold.cpp
@@ -43,7 +43,7 @@
         SkMatrix matrix;
         matrix.reset();
         matrix.setTranslate(WIDTH * .1f, HEIGHT * .1f);
-        matrix.postScale(.8, .8);
+        matrix.postScale(.8f, .8f);
 
         canvas->concat(matrix);