Make SameThreadTask/CrossThreadTask internal and remove platform/Task.h

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'ContentDecryptionModuleResult.h',
@@ skipping 95 matching lines @@
       'SecureTextInput.cpp',
       'SecureTextInput.h',
       'SerializedResource.h',
       'SharedBuffer.cpp',
       'SharedBuffer.h',
       'SharedBufferChunkReader.cpp',
       'SharedBufferChunkReader.h',
       'StorageQuotaCallbacks.h',
       'Supplementable.cpp',
       'Supplementable.h',
-      'Task.h',
       'TaskSynchronizer.cpp',
       'TaskSynchronizer.h',
       'Theme.cpp',
       'Theme.h',
       'ThemeTypes.h',
       'Timer.cpp',
       'Timer.h',
       'TraceEvent.h',
       'TraceEventCommon.h',
       'TracedValue.cpp',
@@ skipping 1195 matching lines @@
       ['OS!="mac"',
         {
           'platform_test_files': [
             'scroll/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }