Use ILLEGAL_MODE macro to silence clang error.

This fixes the following clang error:

../../tests/XfermodeTest.cpp:43:44: error: comparison of constant -1 with expression of type 'SkXfermode::Mode' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
    REPORTER_ASSERT(reporter, reportedMode == -1);
                                  ~~~~~~~~~~~~ ^  ~~

R=vandebo@chromium.org,reed@google.com

Review URL: https://codereview.chromium.org/14180004

git-svn-id: http://skia.googlecode.com/svn/trunk@8692 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/XfermodeTest.cpp b/tests/XfermodeTest.cpp
index 0b7012d..620fbf4 100644
--- a/tests/XfermodeTest.cpp
+++ b/tests/XfermodeTest.cpp
@@ -38,11 +38,11 @@
     }
 
     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();
 }