xwalk_launcher build fix.

Fix linking errors when building xwalk_launcher with component=shared_library.

error: undefined reference to 'AnnotateHappensAfter'
error: undefined reference to 'AnnotateHappensBefore'
error: undefined reference to 'AnnotateHappensAfter'
Merge pull request #2104 from nagineni/build_fix

xwalk_launcher build fix.