Fix the build error due to the rename of package name.

XWalkDefaultWebChromeClient is moved from org.xwalk.core.client to
org.xwalk.core. Change the package name in Cordova as well.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1227
Merge pull request #100 from yongsheng/embeddingapi

Fix the build error due to the rename of package name.