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This CL is a part of the effort to move mock URLRequestJob files
from content/test/net to net/test/url_request/. The reason for this
refactoring is that Cronet will need to use these classes for testing,
so it will be nice if they can be in net/test, as the dependency can
work out nicely.

BUG=

Review URL: https://codereview.chromium.org/541743002

Cr-Commit-Position: refs/heads/master@{#294255}