Add utility method for WebString to base::FilePath conversion

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'ContentDecryptionModuleResult.h',
@@ skipping 1095 matching lines @@
       'WebScreenInfoTest.cpp',
       'WebVectorTest.cpp',
       'animation/AnimationTranslationUtilTest.cpp',
       'animation/TimingFunctionTest.cpp',
       'animation/UnitBezierTest.cpp',
       'animation/CompositorAnimationPlayerTest.cpp',
       'animation/CompositorAnimationTest.cpp',
       'animation/CompositorFloatAnimationCurveTest.cpp',
       'blob/BlobDataTest.cpp',
       'clipboard/ClipboardUtilitiesTest.cpp',
+      'exported/FilePathConversionTest.cpp',
       'fonts/AcceptLanguagesResolverTest.cpp',
       'fonts/CharacterTest.cpp',
       'fonts/FontCacheTest.cpp',
       'fonts/FontDescriptionTest.cpp',
       'fonts/GlyphBufferTest.cpp',
       'fonts/GlyphPageTreeNodeTest.cpp',
       'fonts/OrientationIteratorTest.cpp',
       'fonts/ScriptRunIteratorTest.cpp',
       'fonts/SmallCapsIteratorTest.cpp',
       'fonts/SymbolsIteratorTest.cpp',
@@ skipping 124 matching lines @@
       ['OS!="mac"',
         {
           'platform_test_files': [
             'scroll/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }