Add Widevine CDM support (issue #1631)

- Windows/Mac: Use `--enable-widevine-cdm` command-line flag to
  enable download of CDM binaries via the component updater.
- Linux: Use `--widevide-cdm-path` and `--widevine-cdm-version`
  command-line flags to load CDM binaries that already exist on
  the system.
- A cache-path value is usually required when CDM is enabled.

libcef/browser/context.h

@@ -19,6 +19,10 @@ namespace base {
 class WaitableEvent;
 }
 
+namespace component_updater {
+class ComponentUpdateService;
+}
+
 namespace content {
 class ContentMainRunner;
 }
@@ -63,6 +67,8 @@ class CefContext {
     return print_job_manager_.get();
   }
 
+  component_updater::ComponentUpdateService* component_updater();
+
   CefTraceSubscriber* GetTraceSubscriber();
 
   // Populate the request context settings based on CefSettings and command-
@@ -94,6 +100,9 @@ class CefContext {
 
   // Only accessed on the UI Thread.
   scoped_ptr<printing::PrintJobManager> print_job_manager_;
+
+  // Initially only for Widevine components.
+  scoped_ptr<component_updater::ComponentUpdateService> component_updater_;
 };
 
 // Helper macro that returns true if the global context is in a valid state.