[Prefs] Add CHECKs to debug crbug.com/859477

This change adds CHECKs to help debug crashes originating in
MetricsLogStore.

Bug: 859477
Change-Id: Iab9c770840613cedffdeb24f8b48b0c7580dd825
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599611
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657635}

components/prefs/pref_service.cc

@@ -10,6 +10,7 @@
 
 #include "base/bind.h"
 #include "base/debug/alias.h"
+#include "base/debug/dump_without_crashing.h"
 #include "base/files/file_path.h"
 #include "base/location.h"
 #include "base/logging.h"
@@ -564,13 +565,19 @@ base::Value* PrefService::GetMutableUserPref(const std::string& path,
   // Look for an existing preference in the user store. If it doesn't
   // exist, create a new user preference.
   base::Value* value = nullptr;
-  if (user_pref_store_->GetMutableValue(path, &value))
+  if (user_pref_store_->GetMutableValue(path, &value)) {
+    // TODO(crbug.com/859477): Remove once root cause has been found.
+    if (value->type() != type)
+      base::debug::DumpWithoutCrashing();
     return value;
+  }
 
   // If no user preference exists, clone default value.
   const base::Value* default_value = nullptr;
   pref_registry_->defaults()->GetValue(path, &default_value);
-  DCHECK_EQ(default_value->type(), type);
+  // TODO(crbug.com/859477): Revert to DCHECK once root cause has been found.
+  if (default_value->type() != type)
+    base::debug::DumpWithoutCrashing();
   user_pref_store_->SetValueSilently(path, default_value->CreateDeepCopy(),
                                      GetWriteFlags(pref));
   user_pref_store_->GetMutableValue(path, &value);

components/prefs/scoped_user_pref_update.cc

@@ -25,6 +25,12 @@ base::Value* ScopedUserPrefUpdateBase::GetValueOfType(base::Value::Type type) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   if (!value_)
     value_ = service_->GetMutableUserPref(path_, type);
+
+  // |value_| might be downcast to base::DictionaryValue or base::ListValue,
+  // side-stepping CHECKs built into base::Value. Thus we need to be certain
+  // that the type matches.
+  if (value_)
+    CHECK_EQ(value_->type(), type);
   return value_;
 }