Originial build scripts from Chromium
Enable gyp build for android extensions

1. Remove some irrelevant code to reduce dependencies of other components
2. Add common.gypi
3. Add self-defined source list support to java.gypi
4. Add action of copy js and json to java.gypi for easy packaging
[IAP] Implement query and buy functionalities

Includes following:

1. Two features:
1.1 Query Details of Products
1.2 Buy Products

2. IABHelper Class (by patch):
2.1 Based on Android IAB example
2.2 Added license information
2.3 Added toJson() to SkuDetails
2.4 Added getAllProducts() to Inventory

3. Add interface for IInAppBillingService.aidl

Please refer README for how to use the IAP Extension.

NOTE: This change depends on CL:
https://codereview.chromium.org/317783008/

BUG=https://crosswalk-project.org/jira/browse/XWALK-594
Merge pull request #2 from deqing/iap

[IAP] Add IAP Extension with two features