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Currently the tests are all TEST rather than TEST_F.

Add a test harness (TEST_F) to reduce some boilerplate and encapsulate
some subtle setup code required for overlay scrollers on Mac.

Part of this is to do with ScopedPreferredScrollerStyle: Interleaving
swizzlers like `std::unique_ptr x(new X1); x.reset(new X2);` causes
incorrect behaviour and leakage of the swizzled methods across tests run
in the same process. Specifically, X1's destructor restores the original
methods, but then X2's destructor will restore X1's swizzled methods,
since they were in effect when X2 was constructed. Make it harder for
devs to shoot themselves in the foot with this.

BUG=615948

Review-Url: https://codereview.chromium.org/2442223002
Cr-Commit-Position: refs/heads/master@{#427253}