Remove PluginLoadObserver and related logic, it was only used for NPAPI.

third_party/WebKit/Source/core/html/PluginDocument.cpp

Index: third_party/WebKit/Source/core/html/PluginDocument.cpp
diff --git a/third_party/WebKit/Source/core/html/PluginDocument.cpp b/third_party/WebKit/Source/core/html/PluginDocument.cpp
index dc40f1c589e0c2ed4ec680d099a92f69a9229c59..b10bfbf81946b64efe8a9084a4be079174e91fb4 100644
--- a/third_party/WebKit/Source/core/html/PluginDocument.cpp
+++ b/third_party/WebKit/Source/core/html/PluginDocument.cpp
@@ -135,14 +135,7 @@ void PluginDocumentParser::appendBytes(const char* data, size_t length)
 
 void PluginDocumentParser::finish()
 {
-    if (PluginView* view = pluginView()) {
-        const ResourceError& error = document()->loader()->mainDocumentError();
-        if (error.isNull())
-            view->didFinishLoading();
-        else
-            view->didFailLoading(error);
-        m_embedElement = nullptr;
-    }
+    m_embedElement = nullptr;
     RawDataDocumentParser::finish();
 }