assistant: prepare for deprecating assistant.mojom

This CL removes one of the internal dependency on the public mojom type.

Bug: b:154268518
Change-Id: I68d87c91331cb0347ff0fa23fbb02fe22500f490
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250167Reviewed-by: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779893}

chromeos/services/assistant/assistant_manager_service_impl.cc

@@ -1331,7 +1331,7 @@ void AssistantManagerServiceImpl::ResumeTimer(const std::string& id) {
 void AssistantManagerServiceImpl::NotifyEntryIntoAssistantUi(
     mojom::AssistantEntryPoint entry_point) {
   base::AutoLock lock(last_trigger_source_lock_);
-  last_trigger_source_ = ToTriggerSource(entry_point);
+  last_trigger_source_ = ToTriggerSource(static_cast<int>(entry_point));
 }
 
 std::string AssistantManagerServiceImpl::ConsumeLastTriggerSource() {