[SysApps] Implement the Device Capabilities getCPUInfo()

We first add an callback and function to the IDL to make it generate the Promise
reply serializers, which contains the SystemCPU + error message if applicable.

A function handler for "getCPUInfo" is also needed and it will associate the
message coming from the extension with a class method. It is being registered in
the Instance constructor.

GetCPUInfo::Results::Create is generated from the IDL. This function will
create a message which the promise wrapper will receive.

info->PostResult() has internally the callback ID that maps to a callback ID in
the JavaScript shim, so the message will reach the correct recipient.

In the JavaScript shim, is important to use "this_.addMethodWithPromise" so it will
wrap the Binding Object routing ID automatically to the message and associate it
to the Promise.
[SysApps] Add CPUInfoProvider unit tests

The test basically pings each entry point of the API and check if
the returns values are within a sane range.
[SysApps] Add browsertests for DeviceCapabilities getCPUInfo()

Not a replacement for W3C conformance tests, just checking the basic API
functionality and sanity. The test is crafted with the addition of tests
for the rest of the DeviceCapabilities API in mind, that is why we have
a test list with just one test.