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

third_party/WebKit/Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'AudioOutputDeviceClientImpl.cpp',
       'AudioOutputDeviceClientImpl.h',
       'ChromeClientImpl.cpp',
       'ChromeClientImpl.h',
@@ skipping 170 matching lines @@
       'WebPageImportanceSignals.cpp',
       'WebPagePopupImpl.cpp',
       'WebPagePopupImpl.h',
       'WebPageSerializer.cpp',
       'WebPageSerializerImpl.cpp',
       'WebPageSerializerImpl.h',
       'WebPerformance.cpp',
       'WebPluginContainerImpl.cpp',
       'WebPluginContainerImpl.h',
       'WebPluginDocument.cpp',
-      'WebPluginLoadObserver.cpp',
-      'WebPluginLoadObserver.h',
       'WebPluginScriptForbiddenScope.cpp',
       'WebRange.cpp',
       'WebRemoteFrameImpl.cpp',
       'WebRemoteFrameImpl.h',
       'WebRuntimeFeatures.cpp',
       'WebScopedMicrotaskSuppression.cpp',
       'WebScopedUserGesture.cpp',
       'WebScopedWindowFocusAllowedIndicator.cpp',
       'WebScriptBindings.cpp',
       'WebScriptController.cpp',
@@ skipping 116 matching lines @@
         {
           'web_unittest_files': [
             # Mac uses ScrollAnimatorMac instead of ScrollAnimator.
             'tests/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }