DCHECK on self-assignment of PolicyMap

Add debug assertions against misuses of PolicyMap in self-assignment or
self-modifying constructions.

This CL doesn't bring proper support of such scenarios, but should at
least make it more obvious that they're not supported (by documenting
it and by catching during automated tests).

Bug: none
Change-Id: I9e8a60e48986fd5e3b61b15792b2a24cd6067b9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2233238
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Aya Elsayed <ayaelattar@google.com>
Cr-Commit-Position: refs/heads/master@{#775711}

components/policy/core/common/policy_map.cc

@@ -284,6 +284,8 @@ void PolicyMap::Swap(PolicyMap* other) {
 }
 
 void PolicyMap::CopyFrom(const PolicyMap& other) {
+  DCHECK_NE(this, &other);
+
   Clear();
   for (const auto& it : other)
     Set(it.first, it.second.DeepCopy());
@@ -296,6 +298,8 @@ std::unique_ptr<PolicyMap> PolicyMap::DeepCopy() const {
 }
 
 void PolicyMap::MergeFrom(const PolicyMap& other) {
+  DCHECK_NE(this, &other);
+
   for (const auto& policy_and_entry : other) {
     Entry* current_policy = GetMutableUntrusted(policy_and_entry.first);
     Entry other_policy = policy_and_entry.second.DeepCopy();