fix asserts in SkMulDiv255Round, and add test

tests/MathTest.cpp

Index: tests/MathTest.cpp
===================================================================
--- tests/MathTest.cpp	(revision 8804)
+++ tests/MathTest.cpp	(working copy)
@@ -48,6 +48,32 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
+// test that SkMul16ShiftRound and SkMulDiv255Round return the same result
+static void test_muldivround(skiatest::Reporter* reporter) {
+#if 0
+    // this "complete" test is too slow, so we test a random sampling of it
+
+    for (int a = 0; a <= 32767; ++a) {
+        for (int b = 0; b <= 32767; ++b) {
+            unsigned prod0 = SkMul16ShiftRound(a, b, 8);
+            unsigned prod1 = SkMulDiv255Round(a, b);
+            SkASSERT(prod0 == prod1);
+        }
+    }
+#endif
+
+    SkRandom rand;
+    for (int i = 0; i < 10000; ++i) {
+        unsigned a = rand.nextU() & 0x7FFF;
+        unsigned b = rand.nextU() & 0x7FFF;
+
+        unsigned prod0 = SkMul16ShiftRound(a, b, 8);
+        unsigned prod1 = SkMulDiv255Round(a, b);
+
+        REPORTER_ASSERT(reporter, prod0 == prod1);
+    }
+}
+
 static float float_blend(int src, int dst, float unit) {
     return dst + (src - dst) * unit;
 }
@@ -595,6 +621,8 @@
 
     // disable for now
     if (false) test_blend31();  // avoid bit rot, suppress warning
+
+    test_muldivround(reporter);
 }
 
 #include "TestClassDef.h"