Think implicit promotion is a good idea?

Things like foo.multiply(Sk4f(t,t,t,t)) can just be foo.multiply(t).

BUG=skia:

Review URL: https://codereview.chromium.org/960773005
diff --git a/tests/Sk4xTest.cpp b/tests/Sk4xTest.cpp
index d6e7e51..0985c3b 100644
--- a/tests/Sk4xTest.cpp
+++ b/tests/Sk4xTest.cpp
@@ -83,6 +83,10 @@
     ASSERT_EQ(Sk4i(3,8,15,24),   Sk4i(1,2,3,4).multiply(Sk4i(3,4,5,6)));
 }
 
+DEF_TEST(Sk4x_ImplicitPromotion, r) {
+    ASSERT_EQ(Sk4f(2,4,6,8), Sk4f(1,2,3,4).multiply(2.0f));
+}
+
 DEF_TEST(Sk4x_Comparison, r) {
     ASSERT_EQ(Sk4f(1,2,3,4), Sk4f(1,2,3,4));
     ASSERT_NE(Sk4f(4,3,2,1), Sk4f(1,2,3,4));