Stop importing syncable::WriteTransaction into syncer namespace

In jumbo/unity builds it conflicts with syncer::WriteTransaction defined
in components/sync/syncable/write_transaction.h.

Change-Id: I19a325af6299d955577aec10d36f21b948859035
Reviewed-on: https://chromium-review.googlesource.com/822091Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Commit-Queue: Jens Widell <jl@opera.com>
Cr-Commit-Position: refs/heads/master@{#523502}

components/sync/engine_impl/conflict_resolver.cc

@@ -26,7 +26,6 @@ using syncable::Directory;
 using syncable::Entry;
 using syncable::Id;
 using syncable::MutableEntry;
-using syncable::WriteTransaction;
 
 namespace {
 
@@ -65,7 +64,7 @@ ConflictResolver::ConflictResolver() {}
 
 ConflictResolver::~ConflictResolver() {}
 
-void ConflictResolver::ProcessSimpleConflict(WriteTransaction* trans,
+void ConflictResolver::ProcessSimpleConflict(syncable::WriteTransaction* trans,
                                              const Id& id,
                                              const Cryptographer* cryptographer,
                                              StatusController* status,