[Linux] Add NaCl support

Based on NaCl component work (http://crbug.com/244791) from upstream,
this commit is to add Native Client(aka NaCl) support.
It now can run examples from nacl sdk.
  $ cd <nacl_sdk_dir>/pepper_canary/example
  $ TOOLCHAIN=newlib make serve
  $ xwalk http://localhost:5103

BUG=XWALK-1338
[Android] Fix the potential memory issue in XWalkPreferences

* Use WeakReference instead to make sure the key listener can be GC-ed
* Add unload internal method to remove the listener from the global list

BUG=https://crosswalk-project.org/jira/browse/XWALK-1613
[Android] Format the description, add links and correct typos.

Continue to refine the description of embedding API:
1. Use '<p>' to format the description.
2. Add links to Android API doc.
3. Correct some typos.

BUG=XWALK-1620
[Tizen][Runtime] Fix allow-navigation can not take effect in cross
origin page.

allow-navigation setting should take effect in all web app pages, but in
current implementation, it's only enforced for app local pages. This
patch will expand the scope.
[Android] Show launch screen dialog when no 'image' provided

Previously, when there is no 'image' provided, the launch screen dialog will not be displayed.
It should be displayed, because the backgrounds need to be displayed until the ready_when state.

BUG=XWALK-1635
Merge pull request #1898 from yongsheng/refinedoc

[Android] Format the description, add links and correct typos.
Merge pull request #1894 from hmin/xwalk-preferences

[Android] Fix the potential memory issue in XWalkPreferences
Merge pull request #1904 from takethathe/allow-navigation

[Tizen][Runtime] Fix allow-navigation can not take effect in cross origin page.
Merge pull request #1801 from halton/nacl

[Linux] Add NaCl support
Merge pull request #1905 from kurli/launch-screen-show-bg

[Android] Show launch screen dialog when no 'image' provided
[Android] Use XWalkBaseActivity for each demo in Crosswalk Sample

1. XWalkBaseActivity will control the XWalkView lifecycle, such as onHide, onShow and onDestroy.
   All other demos need to override from this base activity.
2. Add CrosswalkSample to xwalk_builder. (it's not the final solution, will track it with XWALK-1593)

TODO: Removed alert in manifest demo, it will cause crash when onDestroy() is called.
      XWALK-1639

BUG=XWALK-1631