widevine: Use component updater with the Alloy runtime (fixes issue #3149)

Widevine CDM binaries will be downloaded on supported platforms shortly after
application startup. Widevine support will then become available within a few
seconds after successful installation on Windows or after the next application
restart on other platforms. The CDM files will be downloaded to a "WidevineCdm"
directory inside the `CefSettings.user_data_path` directory.

Pass the `--disable-component-update` command-line flag to disable Widevine
download and installation. Pass the `--component-updater=fast-update` command-
line flag to force Widevine download immediately after application startup.

See the related issue for additional usage details.

tests/cefclient/browser/main_context_impl.cc

@@ -7,7 +7,6 @@
 #include <algorithm>
 
 #include "include/cef_parser.h"
-#include "include/cef_web_plugin.h"
 #include "tests/shared/browser/client_app_browser.h"
 #include "tests/shared/common/client_switches.h"
 
@@ -144,16 +143,6 @@ MainContextImpl::MainContextImpl(CefRefPtr<CefCommandLine> command_line,
   if (!use_transparent_painting) {
     browser_background_color_ = background_color_;
   }
-
-  const std::string& cdm_path =
-      command_line_->GetSwitchValue(switches::kWidevineCdmPath);
-  if (!cdm_path.empty()) {
-    // Register the Widevine CDM at the specified path. See comments in
-    // cef_web_plugin.h for details. It's safe to call this method before
-    // CefInitialize(), and calling it before CefInitialize() is required on
-    // Linux.
-    CefRegisterWidevineCdm(cdm_path, nullptr);
-  }
 }
 
 MainContextImpl::~MainContextImpl() {