Adds hook for content embedders to get media::BrowserCdm*.

content/browser/media/cdm/browser_cdm_manager.cc

Index: content/browser/media/cdm/browser_cdm_manager.cc
diff --git a/content/browser/media/cdm/browser_cdm_manager.cc b/content/browser/media/cdm/browser_cdm_manager.cc
index 358f42f8fd90ec78f61cf8b66aad15ab3ae59dd2..72da3079ddf8ea9003d2f5e5372c223ebc79f0bf 100644
--- a/content/browser/media/cdm/browser_cdm_manager.cc
+++ b/content/browser/media/cdm/browser_cdm_manager.cc
@@ -184,7 +184,8 @@ bool BrowserCdmManager::OnMessageReceived(const IPC::Message& msg) {
   return handled;
 }
 
-media::BrowserCdm* BrowserCdmManager::GetCdm(int render_frame_id, int cdm_id) {
+media::BrowserCdm* BrowserCdmManager::GetCdm(int render_frame_id,
+                                             int cdm_id) const {
   DCHECK(task_runner_->RunsTasksOnCurrentThread());
   return cdm_map_.get(GetId(render_frame_id, cdm_id));
 }