[Build] Generate `xwalk_version' from the VERSION file.

One less place that defines the current version, we're on our way to
defining it in only one file.

Borrow Chromium's chrome/tools/build/version.py utility to read the value in
VERSION and get an appropriate version number for it.

Some places in xwalk.gyp depend on the version but are not automatically
regenerated/rebuilt when VERSION changes, but that was also the case before,
and fixing that isn't trivial, so let's defer that part.
Merge pull request #691 from hmin/chrome-version

[Android] Generate chrome version for the frontend url of devtools
Merge pull request #685 from rakuco/version-from-VERSION

[Build] Generate `xwalk_version' from the VERSION file.
[WiDi API] First commit for Presentation API implementation on Android

This commit is the first patch to implement Presentation API spec which
is intended to bring wireless display support into Web. Since the spec
is still in Editor spec and might be changed in future, it is placed
under `navigator.experimental.presentation` namespace.

This patch mainly provides an mock implementation for display available
related API with several test cases. Next step is to add Android port
and requestShow API implementation.

SPEC=http://otcshare.github.io/presentation-spec/index.html
BUG=https://github.com/crosswalk-project/crosswalk/issues/577