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

content/public/test/mock_render_process_host.cc

Index: content/public/test/mock_render_process_host.cc
diff --git a/content/public/test/mock_render_process_host.cc b/content/public/test/mock_render_process_host.cc
index b0514a41df37474c3a9e9ef7b1dd82a061c54238..a9fedb179c1fb05ae0c62e40ef519346b1ac4f44 100644
--- a/content/public/test/mock_render_process_host.cc
+++ b/content/public/test/mock_render_process_host.cc
@@ -248,6 +248,13 @@ void MockRenderProcessHost::SendUpdateValueState(
     unsigned int target, const gpu::ValueState& state) {
 }
 
+#if defined(ENABLE_BROWSER_CDMS)
+media::BrowserCdm* MockRenderProcessHost::GetBrowserCdm(int render_frame_id,
+                                                        int cdm_id) const {
+  return nullptr;
+}
+#endif
+
 void MockRenderProcessHost::FilterURL(bool empty_allowed, GURL* url) {
   RenderProcessHostImpl::FilterURL(this, empty_allowed, url);
 }