Fix compile error when rtc_enable_protobuf is false

When configuring without protobuf this test fails to compile with the error:
perf_test_histogram_writer_no_protobuf.cc:20:1: error: non-void function does not return a value

Bug: None
Change-Id: I8e2676ee4b5284eac08e648fc43bdfc585fc5d64
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/182740
Reviewed-by: Taylor <deadbeef@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Taylor <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32021}

test/testsupport/perf_test_histogram_writer_no_protobuf.cc

@@ -16,6 +16,7 @@ namespace test {
 PerfTestResultWriter* CreateHistogramWriter() {
   RTC_NOTREACHED() << "Cannot run perf tests with rtc_enable_protobuf = false. "
                       "Perf write results as protobufs.";
+  return nullptr;
 }
 
 }  // namespace test