Adds a CancellableTaskFactory and exposes postLoadingTasks to Blink

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 743 matching lines @@
       'network/ResourceResponse.cpp',
       'network/ResourceResponse.h',
       'network/WebSocketHandshakeRequest.cpp',
       'network/WebSocketHandshakeRequest.h',
       'network/WebSocketHandshakeResponse.cpp',
       'network/WebSocketHandshakeResponse.h',
       'plugins/PluginData.cpp',
       'plugins/PluginData.h',
       'plugins/PluginListBuilder.cpp',
       'plugins/PluginListBuilder.h',
+      'scheduler/CancellableTaskFactory.cpp',
+      'scheduler/CancellableTaskFactory.h',
       'scheduler/Scheduler.cpp',
       'scheduler/Scheduler.h',
       'scroll/ProgrammaticScrollAnimator.cpp',
       'scroll/ProgrammaticScrollAnimator.h',
       'scroll/ScrollAnimator.cpp',
       'scroll/ScrollAnimator.h',
       'scroll/ScrollAnimatorNone.cpp',
       'scroll/ScrollAnimatorNone.h',
       'scroll/ScrollTypes.h',
       'scroll/ScrollableArea.cpp',
@@ skipping 162 matching lines @@
       'graphics/ThreadSafeDataTransportTest.cpp',
       'graphics/filters/FilterOperationsTest.cpp',
       'graphics/filters/ImageFilterBuilderTest.cpp',
       'graphics/gpu/DrawingBufferTest.cpp',
       'graphics/test/MockDiscardablePixelRef.h',
       'image-decoders/ImageDecoderTest.cpp',
       'mac/ScrollElasticityControllerTest.mm',
       'network/FormDataTest.cpp',
       'network/HTTPParsersTest.cpp',
       'network/ResourceRequestTest.cpp',
+      'scheduler/CancellableTaskFactoryTest.cpp',
       'scheduler/SchedulerTest.cpp',
       'scroll/ScrollableAreaTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
       'testing/TreeTestHelpers.h',
       'text/BidiResolverTest.cpp',
       'text/DateTimeFormatTest.cpp',
       'text/SegmentedStringTest.cpp',
       'text/UnicodeUtilitiesTest.cpp',
       'transforms/TransformOperationsTest.cpp',
@@ skipping 16 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',
     ],
   },
 }