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TestWebContentsFactory is a class that can create WebContents for unittests,
but can be created on the stack. This gives it the advantage of being able to
be used in tests that are already inheriting from something other than
RenderViewTestHarness.

Review URL: https://codereview.chromium.org/962393002

Cr-Commit-Position: refs/heads/master@{#318740}