[Chromecast] Debug leaked validators in device capabilities.

This will print the keys of validators that were not properly
unregistered before the DeviceCapabilitiesImpl destructor on
debug builds.

Bug: Internal b/138262808
Test: cast_shell_browsertests
Change-Id: Ifda5d7f37844dceebc43902fabac0da86212e95a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715552Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680571}

chromecast/base/device_capabilities_impl.cc

@@ -13,6 +13,7 @@
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/single_thread_task_runner.h"
+#include "base/strings/string_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/values.h"
 #include "chromecast/base/serializers.h"
@@ -145,7 +146,14 @@ DeviceCapabilitiesImpl::DeviceCapabilitiesImpl()
 
 DeviceCapabilitiesImpl::~DeviceCapabilitiesImpl() {
   // Make sure that any registered Validators have unregistered at this point
-  DCHECK(validator_map_.empty());
+  DCHECK(validator_map_.empty())
+      << "Some validators weren't properly unregistered: " << [this] {
+           std::vector<std::string> keys;
+           for (const auto& pair : validator_map_) {
+             keys.push_back(pair.first);
+           }
+           return base::JoinString(keys, ", ");
+         }();
   // Make sure that all observers have been removed at this point
   observer_list_->AssertEmpty();
 }