Add TestMockTimeTaskRunner in base/test.

Multiple implementations of this functionality exist with slight modifications. This CL consolidates them.

BUG=329911
TBR=thestig@chromium.org  # For base/base.gyp changes.

Review URL: https://codereview.chromium.org/823143004

Cr-Commit-Position: refs/heads/master@{#311091}
7 files changed