ExtensionPlatformKeysService: Add a missing return statement

This CL fixes a bug in
ExtensionPlatformKeysService::SignTask::OnCanUseKeyForSigningKnown where
a return statement is missing. That led to calling
ExtensionPlatformKeysService::SignTask::OnCanUseKeyForSigningKnown::DoStep
after the task is finished and deleted.

Bug: 1132926
Change-Id: I0e5be5b4a90e54e045c33e0bfa3568c0737f9272
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435197Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Reviewed-by: Omar Morsi <omorsi@google.com>
Commit-Queue: Omar Morsi <omorsi@google.com>
Cr-Commit-Position: refs/heads/master@{#811355}

chrome/browser/chromeos/platform_keys/extension_platform_keys_service.cc

@@ -354,6 +354,7 @@ class ExtensionPlatformKeysService::SignTask : public Task {
                     platform_keys::Status::kErrorKeyNotAllowedForSigning);
       next_step_ = Step::DONE;
       DoStep();
+      return;
     }
 
     DoStep();