[Extensions] Add a setter callback accessor to trampolines

Prior to this patch we didn't support trampolines with setter
callbacks. This patch enables extensions so can implement an API
to be used like:

"xwalk.sample.onFooChanged = function() { foo("teste"); };"

and use xwalk.sample.onFooChanged as an entry point to xwalk.sample.

BUG=https://crosswalk-project.org/jira/browse/XWALK-835
TEST=./out/Release/xwalk_extensions_browsertest --gtest_filter="MultipleEntryPointsExtension.*"