Add comment and link to bug to regression test.

Bug: 954503
Change-Id: I54d5c45d2d2e3a85c6f88a214f19adaefe5e5a22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577478Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652838}

net/http/http_stream_factory_unittest.cc

@@ -2993,6 +2993,7 @@ TEST_F(HttpStreamFactoryTest, ChangeSocketTag) {
   waiter3.stream()->Close(/* not_reusable = */ true);
 }
 
+// Regression test for https://crbug.com/954503.
 TEST_F(HttpStreamFactoryTest, ChangeSocketTagAvoidOverwrite) {
   SpdySessionDependencies session_deps;
   MockTaggingClientSocketFactory* socket_factory =