[Extensions] Use a vector instead of a map for extension modules

In XWalkModuleSystem, use a vector instead of a map for extension
modules. The number of modules is small and we'll benefit from being
able to easily get sorted iteration later to identify the extension
modules that can be loaded on demand.
Merge pull request #937 from cmarcelo/extension-modules-vector

[Extensions] Use a vector instead of a map for extension modules