build: move libcc patches to electron repo (#14104)

In the GN build, libchromiumcontent is no longer a distinct library, but
merely a container for a set of scripts and patches. Maintaining those
patches in a separate repository is tedious and error-prone, so merge
them into the main repo.

Once this is merged and GN is the default way to build Electron, the
libchromiumcontent repository can be archived.

patches/common/chromium/chrome_key_systems.patch

@@ -0,0 +1,29 @@
+diff --git a/chrome/renderer/media/chrome_key_systems.cc b/chrome/renderer/media/chrome_key_systems.cc
+index f3033337199b..856bc8f0b143 100644
+--- a/chrome/renderer/media/chrome_key_systems.cc
++++ b/chrome/renderer/media/chrome_key_systems.cc
+@@ -14,7 +14,9 @@
+ #include "base/strings/string_split.h"
+ #include "base/strings/utf_string_conversions.h"
+ #include "build/build_config.h"
++#if 0
+ #include "chrome/renderer/chrome_render_thread_observer.h"
++#endif
+ #include "components/cdm/renderer/external_clear_key_key_system_properties.h"
+ #include "components/cdm/renderer/widevine_key_system_properties.h"
+ #include "content/public/renderer/render_thread.h"
+@@ -140,12 +142,14 @@ static void AddExternalClearKey(
+ #if defined(WIDEVINE_CDM_AVAILABLE)
+ // Returns persistent-license session support.
+ EmeSessionTypeSupport GetPersistentLicenseSupport(bool supported_by_the_cdm) {
++#if 0
+   // Do not support persistent-license if the process cannot persist data.
+   // TODO(crbug.com/457487): Have a better plan on this. See bug for details.
+   if (ChromeRenderThreadObserver::is_incognito_process()) {
+     DVLOG(2) << __func__ << ": Not supported in incognito process.";
+     return EmeSessionTypeSupport::NOT_SUPPORTED;
+   }
++#endif
+ 
+   if (!supported_by_the_cdm) {
+     DVLOG(2) << __func__ << ": Not supported by the CDM.";