Add an overload for MemMap::MapAnonymous().

Add an overload that does not take `reuse` or `use_ashmem`
parameters but uses default values (`reuse = false` and
`use_ashmem = true`) to simplify callers.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Iaa44f770dee7e043c3a1d6867dfb0416dec83b25
diff --git a/runtime/runtime_callbacks_test.cc b/runtime/runtime_callbacks_test.cc
index 4c4dcd8..ed0472f 100644
--- a/runtime/runtime_callbacks_test.cc
+++ b/runtime/runtime_callbacks_test.cc
@@ -194,8 +194,7 @@
                                       /* addr */ nullptr,
                                       128 * kPageSize,  // Just some small stack.
                                       PROT_READ | PROT_WRITE,
-                                      false,
-                                      false,
+                                      /* low_4gb */ false,
                                       &error_msg);
   ASSERT_TRUE(stack.IsValid()) << error_msg;