[channel trace test] fix Windows portability tests (#36643)

This fixes breakage triggered by #36434.

Changing the order of the data members in `TraceEvent` caused the total size of the structure to be different, which meant that the long string we were adding was enough to cause 3 events to be evicted instead of 2.

b/339066599

Closes #36643

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36643 from markdroth:channel_trace_test_fix 94586faced
PiperOrigin-RevId: 634505085

test/core/channelz/channel_trace_test.cc

@@ -321,7 +321,7 @@ TEST(ChannelTracerTest, TestEviction) {
 
 TEST(ChannelTracerTest, TestMultipleEviction) {
   ExecCtx exec_ctx;
-  const int kTraceEventSize = GetSizeofTraceEvent();
+  const size_t kTraceEventSize = GetSizeofTraceEvent();
   const int kNumEvents = 5;
   ChannelTrace tracer(kTraceEventSize * kNumEvents);
   std::list<::testing::Matcher<Json>> matchers;
@@ -337,14 +337,12 @@ TEST(ChannelTracerTest, TestMultipleEviction) {
   // At this point the list is full, and each subsequent enntry will cause an
   // eviction. We will now add in a trace event that has a copied string. This
   // uses more memory, so it will cause a double eviciction.
-  tracer.AddTraceEvent(
+  std::string msg(GRPC_SLICE_INLINED_SIZE + 1, 'x');
-      ChannelTrace::Severity::Info,
+  tracer.AddTraceEvent(ChannelTrace::Severity::Info,
-      grpc_slice_from_copied_string(
+                       grpc_slice_from_cpp_string(msg));
-          "long enough string to trigger a multiple eviction"));
   matchers.pop_front();
   matchers.pop_front();
-  matchers.push_back(IsTraceEvent(
+  matchers.push_back(IsTraceEvent(msg, "CT_INFO"));
-      "long enough string to trigger a multiple eviction", "CT_INFO"));
   Json json = tracer.RenderJson();
   ValidateJsonProtoTranslation(json);
   EXPECT_THAT(json, IsChannelTrace(kNumEvents + 1,