[Tizen][Linux] Support the following URI schemes: tel:, sms:, mailto:.

Explanation:

1 Override "content::RuntimeResourceDispatcherHostDelegate::HandleExternalProtocol" in according child class to implement the platform-specific surpporting:
    (1) On TizenIVI, through DBus to invoke corresponding default system application.
    (2) On Linux, use XDG utils.
    (3) On Android, it's been implemented already, keep unchanged.
2 In "XWalkContentBrowserClient::ResourceDispatcherHostCreated" triger the resource_dispatcher_host_delegate to be set.
3 The main reason to implemented the Linux part is to verify our design, the code of function "XDGUtil" is copied  from chrome directly. 
BUG=https://crosswalk-project.org/jira/browse/XWALK-1501
Why use `ALLOW_UNUSED`? To avoid gcc warning? I do not think the unused function para will report error, correct me if I'm wrong.

Besides, upstream code has removed `ALLOW_UNUSED` usage, and use `ALLOW_UNUSED_LOCAL` and `ALLOW_UNUSED_TYPE` instead. https://code.google.com/p/chromium/codesearch#chromium/src/base/compiler_specific.h&q=ALLOW_UNUSED&sq=package:chromium&type=cs