[Android] Add code path for initializing ContentReadbackHandler

ContentReadbackHandler initialization is newly added from chromium
36.0.1985. We need to init it for TextureView path to avoid crash on
destroying.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1887
Merge pull request #164 from hmin/init-readback-handler

[Android] Add code path for initializing ContentReadbackHandler