Avoid using the function name Function in unit_tests.

There is a ::Function template in wtf/Functional.h and if someone else
defines a function with the same name that template will be harder to
use. This is more of a problem in jumbo builds where more code is
visible so that both the definitions becomes visible at the same time.

R=fs@opera.com

Change-Id: I0be2f2e2441f3e5db0049f65e1b4c0be8997d586
Reviewed-on: https://chromium-review.googlesource.com/574028
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#487050}
1 file changed