Revert of Remove WindowFocusAllowedIndicator and associated code.

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationNode.idl',
@@ skipping 1422 matching lines @@
             'page/PopupOpeningObserver.h',
             'page/PrintContext.cpp',
             'page/ScopedPageLoadDeferrer.cpp',
             'page/ScopedPageLoadDeferrer.h',
             'page/SpatialNavigation.cpp',
             'page/TouchAdjustment.cpp',
             'page/TouchAdjustment.h',
             'page/TouchDisambiguation.cpp',
             'page/TouchDisambiguation.h',
             'page/WindowFeatures.cpp',
+            'page/WindowFocusAllowedIndicator.cpp',
             'page/scrolling/ScrollingConstraints.cpp',
             'page/scrolling/ScrollingConstraints.h',
             'page/scrolling/ScrollingCoordinator.cpp',
             'page/scrolling/ScrollingCoordinator.h',
             'paint/BackgroundImageGeometry.cpp',
             'paint/BackgroundImageGeometry.h',
             'paint/BlockFlowPainter.cpp',
             'paint/BlockFlowPainter.h',
             'paint/BlockPainter.cpp',
             'paint/BlockPainter.h',
@@ skipping 2187 matching lines @@
             'rendering/style/RenderStyleTest.cpp',
             'rendering/style/SVGRenderStyleTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }