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

chromecast/browser/media/cma_message_filter_host.cc

Index: chromecast/browser/media/cma_message_filter_host.cc
diff --git a/chromecast/browser/media/cma_message_filter_host.cc b/chromecast/browser/media/cma_message_filter_host.cc
index ae80db13e7a814a76714e020ab2cc36a98f6d570..08242a00cc704350c5ffa305ae3b5b5949d74575 100644
--- a/chromecast/browser/media/cma_message_filter_host.cc
+++ b/chromecast/browser/media/cma_message_filter_host.cc
@@ -17,6 +17,7 @@
 #include "chromecast/media/cma/pipeline/media_pipeline_client.h"
 #include "chromecast/media/cma/pipeline/video_pipeline_client.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/browser/render_process_host.h"
 #include "media/base/bind_to_current_loop.h"
 #include "ui/gfx/geometry/point_f.h"
 #include "ui/gfx/geometry/quad_f.h"
@@ -50,11 +51,13 @@ void UpdateVideoSurfaceHost(int surface_id, const gfx::QuadF& quad) {
 
 }  // namespace
 
-CmaMessageFilterHost::CmaMessageFilterHost(int render_process_id)
+CmaMessageFilterHost::CmaMessageFilterHost(
+    content::RenderProcessHost* render_process_host)
     : content::BrowserMessageFilter(CastMediaMsgStart),
-      process_id_(render_process_id),
+      render_process_host_(render_process_host),
       task_runner_(CmaMessageLoop::GetMessageLoopProxy()),
       weak_factory_(this) {
+  DCHECK(render_process_host);
   weak_this_ = weak_factory_.GetWeakPtr();
 }
 
@@ -164,7 +167,29 @@ void CmaMessageFilterHost::SetCdm(int media_id,
   if (!media_pipeline)
     return;
 
-  FORWARD_CALL(media_pipeline, SetCdm, process_id_, render_frame_id, cdm_id);
+  content::BrowserThread::PostTask(
+      content::BrowserThread::UI, FROM_HERE,
+      base::Bind(&CmaMessageFilterHost::SetCdmOnUiThread,
+                 base::Unretained(this),
+                 media_pipeline,
+                 render_frame_id,
+                 cdm_id));
+}
+
+void CmaMessageFilterHost::SetCdmOnUiThread(
+    MediaPipelineHost* media_pipeline,
+    int render_frame_id,
+    int cdm_id) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+
+  ::media::BrowserCdm* cdm =
+      render_process_host_->GetBrowserCdm(render_frame_id, cdm_id);
+  if (!cdm) {
+    LOG(ERROR) << "Could not find BrowserCdm (" << render_frame_id << ","
+               << cdm_id << ")";
+    return;
+  }
+  FORWARD_CALL(media_pipeline, SetCdm, cdm);
 }
 
 void CmaMessageFilterHost::CreateAvPipe(