Enabling Base Unit Tests.

Updates mojo's Base enough to pass unit tests. Updating Mojo's
version of Base to be entirely consistent with Chromium would be
a significant ordeal, so this CL attempts to add passing Base
unit tests with as few changes to Base as possible.

Pulls in the following from Chromium:

3be75bf76ac061129ec2a9e887d5d50cfd016a56
Updates:
base/i18n/rtl_unittest.cc (Additional changes made manually to pass tests -- removed Windows-specific code).
base/test/icu_test_util.cc
base/test/icu_test_util.h

e89963accb5f3a9d93b8007b22fc79c0120eb794
Updates:
base/files/file_path_unittest.cc

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1445163002 .