Add third_party/widevine/LICENSE

Also move README.chromium up one level as it's expected by
tools/licenses.py.

TBR=tinskip@chromium.org

Bug: 729194
Change-Id: I52551222bf635d5e525d7baa0580243d0a30aa8a
Reviewed-on: https://chromium-review.googlesource.com/1166325
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581625}

third_party/widevine/LICENSE

+Google LLC and its affiliates ("Google") own all legal right, title and
+interest in and to the content decryption module software ("Software") and
+related documentation, including any intellectual property rights in the
+Software. You may not use, modify, sell, or otherwise distribute the Software
+without a separate license agreement with Google.  The Software is not open
+source software.
+
+If you are interested in licensing the Software, please contact www.widevine.com