Complete the layered architecture of data_use_measurement component.

Complete the layered architecture of data_use_measurement component.

Currently the data_use_measurement component is not completely layered
to be compatible with iOS and its layering is not optimal now. Because,
the only usage of //content in the code is to check whether a request is
coming from the user or not. It is desired to move DataUseMeasurement
class to core and have it take in a a DataUseMeasurementDelegate, which
would be an interface defined in the core code with a //content-based
implementation in //components/data_use_measurement/content. Currently
this interface would have only one method:
RequestIsFromUser(net::URLRequest* request).
In the future the interface could expand as needed.
This organization will set up the code better for sharing with iOS
both now and going forward.

BUG=527460