[Extension] Implemented bulk data transmission enhancement test case

Created a test case to test the bulk data transmission enhancement
feature. This extension test case introduced a javascript API to
request data from native side asynchronously, and native side created
the data chunk according to the requested size and post it back to
javascript.

This test case only tests transmission from native to javascript as for
now we've only implemented the enhancement feature in this direction.

This test case has been integrated into xwalk_extensions_browsertest
and can be run by executing:
  out/Debug/xwalk_extensions_browsertest
  --gtest_filter=XWalkExtensionsTest.BulkDataExtension

BUG: https://crosswalk-project.org/jira/browse/XWALK-1377
Merge pull request #2218 from jondong/master

[Extension] Implemented bulk data transmission enhancement test case