Summary:
This tests that URLs point to the TensorFlow mirror (as opposed to the
Bazel mirror) and that they resolve to valid archives.

Test Plan:
Run `./tensorboard/tools/mirror_urls_test.sh`, and note that it passes
with no output. Then apply the following patch to add some bad URLs:

```diff
diff --git a/WORKSPACE b/WORKSPACE
index 3943b0cf..d3975dfe 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -9,6 +9,9 @@ http_archive(
     urls = [
         # tag 0.7.0 resolves to commit 6741f733227dc68137512161a5ce6fcf283e3f58 (2019-02-08 18:37:26 +0100)
         "http://mirror.tensorflow.org/github.com/bazelbuild/bazel-skylib/archive/0.7.0.tar.gz",
+        "http://mirror.tensorflow.org/example.com/nonexistent.txt",
+        "http://mirror.bazel.build/github.com/bazelbuild/bazel-skylib/archive/0.7.0.tar.gz",
+        "https://mirror.bazel.build/github.com/bazelbuild/bazel-skylib/archive/0.7.0.tar.gz",
         "https://github.com/bazelbuild/bazel-skylib/archive/0.7.0.tar.gz",
     ],
 )
```

Re-run the test, and note that it fails with exit code 1 and this text:

```
The following URLs are not properly mirrored:
http://mirror.tensorflow.org/example.com/nonexistent.txt
Googlers, see http://b/133880558 for further instructions.

The following URLs point to the legacy Bazel mirror:
WORKSPACE:13:        "http://mirror.bazel.build/github.com/bazelbuild/bazel-skylib/archive/0.7.0.tar.gz",
WORKSPACE:14:        "https://mirror.bazel.build/github.com/bazelbuild/bazel-skylib/archive/0.7.0.tar.gz",
Please update them to use http://mirror.tensorflow.org/ instead.

```

wchargin-branch: mirror-urls-test

wchargin-source: 3f549e6ed919d47770f7977cfb433116c176875a
wchargin-branch: mirror-urls-test

wchargin-branch: mirror-urls-test

# Conflicts:
#	.travis.yml

wchargin-source: a3c563dc262c3ecf8567816297cf54b0dc4df065
wchargin-branch: mirror-urls-test

wchargin-source: 9a50cec7bef035729fc363da2c3c7f63d6b83834
wchargin-branch: mirror-urls-test

wchargin-source: 70eeb29a52fc768ff34982e07060cd83d7d8a6ee
wchargin-branch: mirror-urls-test