Introduce EditingTestBase to share code among test files for editing

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 3865 matching lines @@
             'dom/DocumentTest.cpp',
             'dom/MainThreadTaskRunnerTest.cpp',
             'dom/NthIndexCacheTest.cpp',
             'dom/PositionTest.cpp',
             'dom/RangeTest.cpp',
             'dom/ScriptRunnerTest.cpp',
             'dom/TreeScopeStyleSheetCollectionTest.cpp',
             'dom/TreeScopeTest.cpp',
             'dom/shadow/ComposedTreeTraversalTest.cpp',
             'editing/CompositionUnderlineRangeFilterTest.cpp',
+            'editing/EditingTestBase.cpp',
+            'editing/EditingTestBase.h',
             'editing/FrameSelectionTest.cpp',
             'editing/GranularityStrategyTest.cpp',
             'editing/InputMethodControllerTest.cpp',
             'editing/iterators/TextIteratorTest.cpp',
             'editing/StyledMarkupSerializerTest.cpp',
             'editing/SurroundingTextTest.cpp',
             'editing/VisiblePositionTest.cpp',
             'editing/VisibleSelectionTest.cpp',
             'events/EventPathTest.cpp',
             'fetch/CachingCorrectnessTest.cpp',
@@ skipping 66 matching lines @@
             'style/OutlineValueTest.cpp',
             'style/SVGComputedStyleTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }