Check if logging for a severity is enabled at the call site.

This does mean that each call site becomes slightly more expensive
in terms of binary size, but until we have a better way to mitigate
the perf impact, I think we'll have to live with it.

We could also consider migrating LS_VERBOSE over to RTC_DLOG.

Bug: webrtc:11968
Change-Id: Ib16f1109366ffaa88b8df28ebfa5bc3b539f691f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/184922
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32153}

rtc_base/logging_unittest.cc

diff --git a/rtc_base/logging_unittest.cc b/rtc_base/logging_unittest.cc
index a66f8b5..6bb20ab 100644
--- a/rtc_base/logging_unittest.cc
+++ b/rtc_base/logging_unittest.cc
@@ -359,5 +359,19 @@
   stream.Close();
 }
 
+TEST(LogTest, NoopSeverityDoesNotRunStringFormatting) {
+  if (!LogMessage::IsNoop(LS_VERBOSE)) {
+    RTC_LOG(LS_WARNING) << "Skipping test since verbose logging is turned on.";
+    return;
+  }
+  bool was_called = false;
+  auto cb = [&was_called]() {
+    was_called = true;
+    return "This could be an expensive callback.";
+  };
+  RTC_LOG(LS_VERBOSE) << "This should not be logged: " << cb();
+  EXPECT_FALSE(was_called);
+}
+
 }  // namespace rtc
-#endif
+#endif  // RTC_LOG_ENABLED()