Blink Plugins: Remove Shadow DOM plugin placeholder tests.

Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'ChromeClientImpl.cpp',
       'ChromeClientImpl.h',
       'ColorChooserPopupUIController.cpp',
       'ColorChooserPopupUIController.h',
@@ skipping 242 matching lines @@
       'mac/WebSubstringUtil.mm',
       'painting/PaintAggregator.cpp',
       'painting/PaintAggregator.h',
       'win/WebFontRendering.cpp',
     ],
     'web_unittest_files': [
       'AssociatedURLLoaderTest.cpp',
       'ExternalPopupMenuTest.cpp',
       'LinkHighlightImplTest.cpp',
       'PageOverlayTest.cpp',
-      'PluginPlaceholderImplTest.cpp',
       'WebDragDataTest.cpp',
       'WebEmbeddedWorkerImplTest.cpp',
       'WebNodeTest.cpp',
       'WebElementTest.cpp',
       # FIXME: Move the tests from web/tests/ to appropriate places.
       # crbug.com/353585
       'tests/ActivityLoggerTest.cpp',
       'tests/ChromeClientImplTest.cpp',
       'tests/CustomEventTest.cpp',
       'tests/DocumentLoaderTest.cpp',
@@ skipping 49 matching lines @@
         {
           'web_unittest_files': [
             # Mac uses ScrollAnimatorMac instead of ScrollAnimatorNone.
             'tests/ScrollAnimatorNoneTest.cpp',
           ],
         }
       ],
     ],
   },
 }