[Android] Fix the render issue introduced by single process mode.

* In Chromium, single process mode is only for Android WebView, ContentShell and
its derivatives only go to the multiple process mode.
* The context design is only for multiple process mode(ContentShell/XWalk), if we
force running it with single process mode, there are issues (black regions for
video/canvas/webgl layers, and black screen for stop/restart actions). The reason
is that Android port has browser-side compositor, and the shared context in
browser-side compositor has conflicts with renderer's contexts if running in single
process mode.
* To run ContentShell/XWalk in single process mode, we have to resolve the shared
context issue between renderer/browser.

BUG=https://github.com/crosswalk-project/crosswalk/issues/516
Merge pull request #67 from sqliu/fix-shared-context

[Android] Fix the render issue introduced by single process mode.