This is a (less ambitious) re-land of http://crrev.com/2852613002 which
was reverted due to content_browsertests crashes on ChromeOS. That CL is
a re-land of http://crrev.com/2779193002 which was reverted due to build
errors on Mac.

Instead of rolling Google Test to 1.8.0, this CL uses the last commit
before a breaking change was introduced. Details about the breakage are
in https://crbug.com/630705#c27 and https://crbug.com/630705#c28. It is
expected that rolling Google Test from there to the next usable version
will be much less disruptive.

GoogleTest (gtest) and GoogleMock (gmock) are now hosted into the same
googletest repository. In order to cope with this, the googletest
repository is now sourced at third_party/googletest.

The file/directory layout of Google Test is not yet considered stable.
To minimize disruption while Google Test stabilizes, Chromium code will
be insulated from third_party/googletest.

* testing/gtest/include/gtest/ and testing/gmock/include/gmock have
  been populated with headers that forward into the appropriate locations of
  third_party/googletest

* testing/BUILD.gn has been populated with the targets
  //testing/gtest(:gtest_main) and //testing/gmock(:gmock_main), which
  depend on the appropriate //third_party/googletest targets.

All Chromium code should keep depending on the targets and headers
in testing/{gtest,gmock} for now.

BUG=630705
TESTED=out/Default/content_browsertests --gtest_filter=MojoTest.Init
TESTED=ninja -C out/Default/ ced_unittests && ninja -C out/Default
TBR=rkc, dpranke

Review-Url: https://codereview.chromium.org/2856383002
Cr-Commit-Position: refs/heads/master@{#469551}