[BuildFix] Add an explicit dependency on webkit_support.gyp:glue_child

This fix is to add an explicit dependency on webkit_support.gyp:`glue_child` to
`xwalk_runtime` target, otherwise, xwalk will throw an error message
saying 'HeapProfilerStart symbol is not found' at runtime.

BUG=https://github.com/crosswalk-project/crosswalk/issues/859
Merge pull request #870 from hmin/build-fix

[BuildFix] Add an explicit dependency on webkit_support.gyp:glue_child