Proactively do BeginFrame when pending tree is activated and no input events are consumed

This is a continuing work on https://github.com/crosswalk-project/chromium-crosswalk/pull/52 after the code base has been updated to align with Chromium 29.

Since the architecture is changed in the upgrade to Chromium 29, a different approach is used in this  patch and it can get the similar performance improvement with the one at https://github.com/crosswalk-project/chromium-crosswalk/pull/52. 

Chromium upstream(Google guys) has merged this logic into their deadline-scheduling approach and can get similar performance gain. The related patch has been landed into chromium 31.0.1639.1. It is now enabled by default for android, and for other platforms you need to enable it from about://flags or add command line flags "--enable-deadline-scheduling". The details can be found at http://code.google.com/p/chromium/issues/detail?id=284247 and http://code.google.com/p/chromium/issues/detail?id=243461.

It is better to merge this logic alone to Crosswalk to get the performance gain at present, a future rebase to M31 can get the whole updates on Chrome Compositor.