[Extensions] Add a setter callback accessor to trampolines

Prior to this patch we weren't supporting trampolines with setter
callbacks. This patch enables extensions so can implement an API
to be used like:

"window.onFooChanged = function() { foo("teste"); };"

and use window.onFooChanged as an entry point to an extension called
xwalk.sample, for instance.

BUG=https://crosswalk-project.org/jira/browse/XWALK-835
TEST=./out/Release/xwalk_extensions_browsertest --gtest_filter="MultipleEntryPointsExtension.*"
Merge pull request #1479 from jeez/lazy-load-setter-callback

[Extensions] Add a setter callback accessor to trampolines