remove SkScalarMul from public headers

BUG=skia:

Review URL: https://codereview.chromium.org/132453008

git-svn-id: http://skia.googlecode.com/svn/trunk@13189 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/core/SkScalar.h b/include/core/SkScalar.h
index 4e0d91b..b9256ba 100644
--- a/include/core/SkScalar.h
+++ b/include/core/SkScalar.h
@@ -202,7 +202,7 @@
 */
 static inline SkScalar SkScalarInterp(SkScalar A, SkScalar B, SkScalar t) {
     SkASSERT(t >= 0 && t <= SK_Scalar1);
-    return A + SkScalarMul(B - A, t);
+    return A + (B - A) * t;
 }
 
 /** Interpolate along the function described by (keys[length], values[length])