Move DragCaretController class to its own file

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 1260 matching lines @@
             'editing/CompositionUnderline.h',
             'editing/CompositionUnderlineRangeFilter.cpp',
             'editing/CompositionUnderlineRangeFilter.h',
             'editing/CreateLinkCommand.cpp',
             'editing/CreateLinkCommand.h',
             'editing/DOMSelection.cpp',
             'editing/DOMSelection.h',
             'editing/DeleteFromTextNodeCommand.cpp',
             'editing/DeleteFromTextNodeCommand.h',
             'editing/DeleteSelectionCommand.cpp',
+            'editing/DragCaretController.cpp',
+            'editing/DragCaretController.h',
             'editing/EditCommand.cpp',
             'editing/EditingBehavior.cpp',
             'editing/EditingBehavior.h',
             'editing/EditingStrategy.cpp',
             'editing/EditingStrategy.h',
             'editing/EditingStyle.cpp',
             'editing/Editor.cpp',
             'editing/EditorCommand.cpp',
             'editing/EditorKeyBindings.cpp',
             'editing/EphemeralRange.cpp',
@@ skipping 2501 matching lines @@
             'style/OutlineValueTest.cpp',
             'style/SVGComputedStyleTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }