Fix test OneByteArrayJoin.

The test OneByteArrayJoin failed on MIPS64 in debug mode after
https://chromium-review.googlesource.com/c/489946/ with error messages
"allocation failure GC in old space requested" and
"Fatal javascript OOM in CALL_AND_RETRY_LAST". Successful test execution
is possible on two ways: with flag --max_old_space_size=7 or with
modification in the test (constraints.set_max_old_space_size(7)).

TEST=cctest/test-strings/OneByteArrayJoin
BUG=

Review-Url: https://codereview.chromium.org/2907803002
Cr-Commit-Position: refs/heads/master@{#45548}

test/cctest/test-strings.cc

@@ -1301,7 +1301,7 @@ UNINITIALIZED_TEST(OneByteArrayJoin) {
   v8::Isolate::CreateParams create_params;
   // Set heap limits.
   create_params.constraints.set_max_semi_space_size(1);
-  create_params.constraints.set_max_old_space_size(6);
+  create_params.constraints.set_max_old_space_size(7);
   create_params.array_buffer_allocator = CcTest::array_buffer_allocator();
   v8::Isolate* isolate = v8::Isolate::New(create_params);
   isolate->Enter();