Don't needlessly include <iostream>.

iostream contains a static initializer and generally should be frowned upon.
It also makes presubmit fire when touching this file.  (But after removing
this, they complain about streams, so it doesn't really solve that problem.)

BUG=none

Review URL: https://codereview.chromium.org/1626413002

Cr-Commit-Position: refs/heads/master@{#371496}

third_party/WebKit/Source/core/css/CSSValueTestHelper.h

@@ -39,7 +39,7 @@
 #include "core/css/CSSPrimitiveValue.h"
 #include "core/css/CSSValue.h"
 
-#include <iostream>
+#include <ostream>
 
 namespace testing {
 namespace internal {