[BuildFix] Fix build for -Dcomponent=shared_library

After rebasing to Chromium M31, Crosswalk stopped building when
gyp_xwalk is invoked with '-D component=shared_library' [1], throwing
several linking errors. This patch fixes the build by adding missing
dependencies to xwalk.gyp.

BUG=https://github.com/crosswalk-project/crosswalk/issues/859

[1] https://code.google.com/p/chromium/wiki/LinuxFasterBuilds#Dynamically_link