Move CHECK to DCHECK.

GetIsGcMarking can be hot when running under the interpreter. Remove the
CHECK.

Test: test.py
Change-Id: Id145bc8dd55aea416aa95265f7af15b573abc948
diff --git a/runtime/thread.h b/runtime/thread.h
index 95d06cf..8bef83f 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -1063,7 +1063,7 @@
   }
 
   bool GetIsGcMarking() const {
-    CHECK(gUseReadBarrier);
+    DCHECK(gUseReadBarrier);
     return tls32_.is_gc_marking;
   }