Merge pull request #959 from cmarcelo/extension-use-string

[Extensions] Remove GetJavaScriptAPI() virtual function
Move resource_ids file outside runtime

The resource_ids file is an index for all the "grd" files that describe
resources (strings, images, data) we bundle with Crosswalk.

We want to provide resources for other parts of Crosswalk, so move this
index outside runtime/.
[Dialog] Use resource bundle instead of xwalk_js2c target

This patch changes our xwalk.experimental.dialog to use ResourceBundle
instead of our previous solution of generating a C source file to be
included.
[Extensions] Use ResourceBundle for storing JS instead of xwalk_js2c
[Runtime] Use ResourceBundle instead of xwalk_js2c.gypi
[Application] Use ResourceBundle instead of xwalk_js2c.gypi
[Extensions] Remove xwalk_js2c.gypi

This GYP file and the corresponding script are not used anymore. We now
bundle the JavaScript code using the resources system.