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/webview_reattach.patch

@@ -0,0 +1,16 @@
+diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
+index b2d1e63..bc2cf9a 100644
+--- a/content/browser/web_contents/web_contents_impl.cc
++++ b/content/browser/web_contents/web_contents_impl.cc
+@@ -4562,6 +4562,11 @@ void WebContentsImpl::NotifyViewSwapped(RenderViewHost* old_host,
+   for (auto& observer : observers_)
+     observer.RenderViewHostChanged(old_host, new_host);
+ 
++  // If this is an inner WebContents that has swapped views, we need to reattach
++  // it to its outer WebContents.
++  if (node_.outer_web_contents())
++    ReattachToOuterWebContentsFrame();
++
+   // Ensure that the associated embedder gets cleared after a RenderViewHost
+   // gets swapped, so we don't reuse the same embedder next time a
+   // RenderViewHost is attached to this WebContents.