Unify the three places that patch font loading for PDFium on Windows.

content/plugin/plugin_thread.h

Index: content/plugin/plugin_thread.h
diff --git a/content/plugin/plugin_thread.h b/content/plugin/plugin_thread.h
index 11acafc79c1b104da9dc79f4ed12e0e584301c2d..8ad115e21c9c703187ade90546fbf09985c0c49e 100644
--- a/content/plugin/plugin_thread.h
+++ b/content/plugin/plugin_thread.h
@@ -8,7 +8,7 @@
 #include "base/files/file_path.h"
 #include "base/native_library.h"
 #include "build/build_config.h"
-#include "content/child/child_thread.h"
+#include "content/child/child_thread_impl.h"
 #include "content/child/npapi/plugin_lib.h"
 #include "content/plugin/plugin_channel.h"
 
@@ -22,7 +22,7 @@ class BlinkPlatformImpl;
 // The PluginThread class represents a background thread where plugin instances
 // live.  Communication occurs between WebPluginDelegateProxy in the renderer
 // process and WebPluginDelegateStub in this thread through IPC messages.
-class PluginThread : public ChildThread {
+class PluginThread : public ChildThreadImpl {
  public:
   PluginThread();
   ~PluginThread() override;