Introduce WebDataConsumerHandle.

Define an abstract class which whose implementation will be in content/renderer.
This class is used by fetch API with Streams.

BUG=418879

Review URL: https://codereview.chromium.org/626803002

git-svn-id: svn://svn.chromium.org/blink/trunk@184493 bbb929c8-8fbe-4397-9dbb-9b2b20218538