[Sync] Check typed_url_sync_bridge_ pointer before dereferencing

Instantiation of typed_url_sync_bridge_ in HistoryBackend is controlled by Finch
experiment, when experiment is disabled typed_url_sync_bridge_ is nullptr. In my
previous change (https://crrev.com/c/815880) I introduced call to notify
TypedURLSyncBridge about SQL error, but the call happens regardless if
experiment is enabled or not. This caused crashes described in corresponding
bug.

In this change I check typed_url_sync_bridge_ to be valid before dereferencing
it to call a function.

BUG=796138
R=skym@chromium.org

Change-Id: I08b45479c6d6bce42709ccaad76491ca76bfb232
Reviewed-on: https://chromium-review.googlesource.com/849192Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526840}

components/history/core/browser/history_backend.cc

@@ -2529,7 +2529,8 @@ void HistoryBackend::DatabaseErrorCallback(int error, sql::Statement* stmt) {
 
     // Notify SyncBridge about storage error. It will report failure to sync
     // engine and stop accepting remote updates.
-    typed_url_sync_bridge_->OnDatabaseError();
+    if (typed_url_sync_bridge_)
+      typed_url_sync_bridge_->OnDatabaseError();
 
     // Don't just do the close/delete here, as we are being called by |db| and
     // that seems dangerous.

components/history/core/browser/history_backend_unittest.cc

@@ -51,6 +51,7 @@
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/skia/include/core/SkBitmap.h"
+#include "third_party/sqlite/sqlite3.h"
 #include "ui/gfx/codec/png_codec.h"
 #include "url/gurl.h"
 
@@ -3839,6 +3840,15 @@ TEST_F(HistoryBackendTest, DeleteFTSIndexDatabases) {
   EXPECT_TRUE(base::PathExists(db2_actual));  // Symlinks shouldn't be followed.
 }
 
+// Tests that calling DatabaseErrorCallback doesn't cause crash. (Regression
+// test for https://crbug.com/796138)
+TEST_F(HistoryBackendTest, DatabaseError) {
+  EXPECT_EQ(nullptr, backend_->GetTypedURLSyncBridge());
+  backend_->DatabaseErrorCallback(SQLITE_CORRUPT, nullptr);
+  // Run loop to let any posted callbacks run before TearDown().
+  base::RunLoop().RunUntilIdle();
+}
+
 // Common implementation for the two tests below, given that the only difference
 // between them is the type of the notification sent out.
 void InMemoryHistoryBackendTest::TestAddingAndChangingURLRows(