Move PopupMenuClient.h to core/html/forms

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 83 matching lines @@
       'PlatformResourceLoader.cpp',
       'PlatformResourceLoader.h',
       'PlatformScreen.cpp',
       'PlatformScreen.h',
       'PlatformThreadData.cpp',
       'PlatformThreadData.h',
       'PlatformTouchEvent.h',
       'PlatformTouchPoint.h',
       'PlatformWheelEvent.h',
       'PopupMenu.h',
-      'PopupMenuClient.h',
       'PopupMenuStyle.h',
       'Prerender.cpp',
       'Prerender.h',
       'PrerenderClient.h',
       'PurgeableVector.cpp',
       'PurgeableVector.h',
       'RefCountedSupplement.h',
       'ScriptForbiddenScope.cpp',
       'ScriptForbiddenScope.h',
       'SecureTextInput.cpp',
@@ skipping 851 matching lines @@
       'graphics/ImageFrameGeneratorTest.cpp',
       'graphics/ImageLayerChromiumTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
   },
 }