Revert "Do not create proto database provider for incognito profile."

This reverts commit e69e60b1.

Reason for revert: browser crash, see crbug.com/1110091 for details.

TBR=ssid, nyquist
BUG=1110091, 1109349

Original change's description:
> Do not create proto database provider for incognito profile.
>
> This is to verify that we currently do not create any database in
> incognito profile. Support for in-memory database for incognito will be
> added in future.
>
> BUG= 1109349
>
> Change-Id: I7dbda9226c207315a4b6b807fac2791f45b89725
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303436
> Reviewed-by: Bo <boliu@chromium.org>
> Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
> Commit-Queue: ssid <ssid@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#791419}

TBR=nyquist@chromium.org,boliu@chromium.org,ssid@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1109349
Change-Id: If66c18de62a5e983ed08040d990cd5f5dd77ac45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321144
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792080}

content/browser/storage_partition_impl.cc

@@ -1640,8 +1640,6 @@ NativeIOContext* StoragePartitionImpl::GetNativeIOContext() {
 
 leveldb_proto::ProtoDatabaseProvider*
 StoragePartitionImpl::GetProtoDatabaseProvider() {
-  if (is_in_memory_)
-    return nullptr;
   if (!proto_database_provider_) {
     proto_database_provider_ =
         std::make_unique<leveldb_proto::ProtoDatabaseProvider>(partition_path_);