Expose platform thread id on WebThread

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 123 matching lines @@
       'Timer.h',
       'TraceEvent.h',
       'TracedValue.cpp',
       'TracedValue.h',
       'UserGestureIndicator.cpp',
       'UserGestureIndicator.h',
       'URLPatternMatcher.cpp',
       'URLPatternMatcher.h',
       'UUID.cpp',
       'UUID.h',
+      'WebThread.cpp',
       'Widget.cpp',
       'Widget.h',
       'WindowsKeyboardCodes.h',
       'animation/AnimationUtilities.h',
       'animation/AnimationValue.h',
       'animation/KeyframeValueList.cpp',
       'animation/KeyframeValueList.h',
       'animation/TimingFunction.cpp',
       'animation/TimingFunction.h',
       'animation/UnitBezier.h',
@@ skipping 804 matching lines @@
       'graphics/ImageLayerChromiumTest.cpp',
       'graphics/OpaqueRectTrackingContentLayerDelegateTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
   },
 }