Add unittest for OpenTypeSanitizer

Since TestingPlatformSupport::unitTestSupport() doesn't implement required methods(webKitRootDir and readFromFile), I needed to put this test in platform_web_unittest_files.

BUG=371267

Review URL: https://codereview.chromium.org/316343004

git-svn-id: svn://svn.chromium.org/blink/trunk@176593 bbb929c8-8fbe-4397-9dbb-9b2b20218538