Explicitly ignore another unused result of make_unique<>

Recent versions of libc++ mark make_unique<>'s result as [[nodiscard]]:
llvm/llvm-project#168483

This place intentionally returns make_unique<>'s result, so
explicitly annotate it as such.

Previously:
https://chromium-review.googlesource.com/c/chromium/src/+/7199487
This instance here is android-only, which is why it wasn't part of
that CL.

No behavior change.

Bug: none
Change-Id: I3293f65934d34cfd63795a6b34008aa4a8d78237
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7207549
Reviewed-by: Hans Wennborg <[email protected]>
Commit-Queue: Hans Wennborg <[email protected]>
Auto-Submit: Nico Weber <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1550606}
NOKEYCHECK=True
GitOrigin-RevId: 1383ff0a2548cf347e245d797e1144885da2231f

Author: nico

debug/allocation_trace_unittest.cc

@@ -332,7 +332,7 @@ TEST_F(AllocationTraceRecorderTest, VerifyWithHooks) {
   dispatcher.InitializeForTesting(&subject_under_test);
 
   // Perform an allocation and free.
-  std::make_unique<std::string>(
+  std::ignore = std::make_unique<std::string>(
       "Just enforce an allocation and free to trigger notification of the "
       "subject under test. Hopefully this string is long enough to bypass any "
       "small string optimizations that the STL implementation might use.");