Revert "[Android] Enable multiple threads for rasterization"

This reverts commit 7eb4b60ac303d6916958c3b400bd9863a848040b.

Crosswalk will be crashed in DEBUG mode since there is a regression happened
in Chromium 34 if multiple threaded rasterization is enabled.

Conflicts:

	runtime/browser/xwalk_browser_main_parts_android.cc
Merge pull request #1609 from hmin/revert-mt-rasterization

Revert "[Android] Enable multiple threads for rasterization"