Use Position instead of VisiblePosition for SurroundingText.

Source/core/core.gypi

 {
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationPlayer.idl',
             'animation/AnimationNode.idl',
             'animation/AnimationTimeline.idl',
             'animation/Timing.idl',
@@ skipping 3364 matching lines @@
             'css/parser/SizesAttributeParserTest.cpp',
             'css/parser/MediaConditionTest.cpp',
             'css/resolver/FontBuilderTest.cpp',
             'dom/ActiveDOMObjectTest.cpp',
             'dom/DOMImplementationTest.cpp',
             'dom/DocumentMarkerControllerTest.cpp',
             'dom/DocumentTest.cpp',
             'dom/MainThreadTaskRunnerTest.cpp',
             'dom/RangeTest.cpp',
             'dom/TreeScopeTest.cpp',
+            'editing/SurroundingTextTest.cpp',
             'editing/TextIteratorTest.cpp',
             'editing/VisibleSelectionTest.cpp',
             'fetch/CachingCorrectnessTest.cpp',
             'fetch/ImageResourceTest.cpp',
             'fetch/MemoryCacheTest.cpp',
             'fetch/RawResourceTest.cpp',
             'fetch/ResourceFetcherTest.cpp',
             'frame/ImageBitmapTest.cpp',
             'html/HTMLDimensionTest.cpp',
             'html/HTMLLinkElementSizesAttributeTest.cpp',
             'html/LinkRelAttributeTest.cpp',
             'html/TimeRangesTest.cpp',
             'html/parser/HTMLParserThreadTest.cpp',
             'html/parser/HTMLSrcsetParserTest.cpp',
             'html/track/vtt/BufferedLineReaderTest.cpp',
             'html/track/vtt/VTTScannerTest.cpp',
             'loader/MixedContentCheckerTest.cpp',
             'page/NetworkStateNotifierTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }