Use ILLEGAL_MODE macro to silence clang error.

tests/XfermodeTest.cpp

Index: tests/XfermodeTest.cpp
diff --git a/tests/XfermodeTest.cpp b/tests/XfermodeTest.cpp
index 0b7012d41711c8d3bd442d2ad4192ac250dd60d6..620fbf4c93abe5224e119a3602e164e752fad61b 100644
--- a/tests/XfermodeTest.cpp
+++ b/tests/XfermodeTest.cpp
@@ -38,11 +38,11 @@ static void test_asMode(skiatest::Reporter* reporter) {
     }
 
     SkXfermode* bogusXfer = new SkProcXfermode(bogusXfermodeProc);
-    SkXfermode::Mode reportedMode = (SkXfermode::Mode) -1;
+    SkXfermode::Mode reportedMode = ILLEGAL_MODE;
     REPORTER_ASSERT(reporter, !bogusXfer->asMode(&reportedMode));
-    REPORTER_ASSERT(reporter, reportedMode == -1);
+    REPORTER_ASSERT(reporter, reportedMode == ILLEGAL_MODE);
     REPORTER_ASSERT(reporter, !SkXfermode::IsMode(bogusXfer, &reportedMode));
-    REPORTER_ASSERT(reporter, reportedMode == -1);
+    REPORTER_ASSERT(reporter, reportedMode == ILLEGAL_MODE);
     bogusXfer->unref();
 }