googletest: Add License File to README.chromium.

Set it to "NOT_SHIPPED" since googletest is only used for running tests. Not
setting it makes //tools/licenses.py complain.

Change-Id: I5146c7d6a7a64bcd19a194246bc24fcc47f12f32
Reviewed-on: https://chromium-review.googlesource.com/597907Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#491371}

third_party/googletest/README.chromium

@@ -3,6 +3,7 @@ Short Name: googletest
 URL: https://github.com/google/googletest.git
 Version: 1.8.0.git-8c7f93fedaca1b0158e67af0f5dd63a044066eab
 License: BSD
+License File: NOT_SHIPPED
 Security critical: no
 
 Google Test is imported as-is, to facilitate version bumping. However, the