Don't store session storage to disk in incognito mode.

SessionStorageContextMojo uses an in-memory database if base::nullopt is
passed as path for the DB, but DOMStorageContextWrapper never actually
passed nullopt, so we'd still always be writing session storage to disk.

Bug: 859010
Change-Id: I3555cf3961adcfe6766fdb46d71699490e01c371
Reviewed-on: https://chromium-review.googlesource.com/1120440
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571592}

content/browser/dom_storage/dom_storage_context_wrapper.cc

@@ -153,7 +153,9 @@ DOMStorageContextWrapper::DOMStorageContextWrapper(
 
   if (base::FeatureList::IsEnabled(features::kMojoSessionStorage)) {
     mojo_session_state_ = new SessionStorageContextMojo(
-        mojo_task_runner_, connector, local_partition_path,
+        mojo_task_runner_, connector,
+        profile_path.empty() ? base::nullopt
+                             : base::make_optional(local_partition_path),
         std::string(kSessionStorageDirectory));
   }