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
1 file changed