Remove CHECK in WebContentsObserverSanityChecker::DidStopLoading

The check causes low-level browser_test flake across a wide variety of
test suites, usually due to check failures during test suite teardown.
This is test-only code, so relax the check until the root cause can be
investigated further.

Bug: 466089, 871010, 913006, 915360, 919347
Test: browser_tests
Change-Id: Ife2ab6a6d260471f79873b6f0fd2914a4c7bb71d
Reviewed-on: https://chromium-review.googlesource.com/c/1403353Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622067}

content/test/web_contents_observer_sanity_checker.cc

@@ -316,7 +316,8 @@ void WebContentsObserverSanityChecker::DidStartLoading() {
 }
 
 void WebContentsObserverSanityChecker::DidStopLoading() {
-  CHECK(is_loading_);
+  // TODO(crbug.com/466089): Add back CHECK(is_loading_). The CHECK was removed
+  // because of flaky failures during browser_test shutdown.
   CHECK(!web_contents()->IsLoading());
   is_loading_ = false;
 }