Add license files for auto and guava.

From https://chromium-review.googlesource.com/c/chromium/src/+/750645
Chromium started depending on auto (which in turns depends on guava)
as an errorprone plugin.

It looks like this this affects only libwebrtc, since it's the only target of
type "dist_jar". Not sure why.

R=mbonadei@webrtc.org, phoglund@webrtc.org

Bug: None
Change-Id: Ib332483e46744f96db6137d243a118b98fa77b91
Reviewed-on: https://webrtc-review.googlesource.com/20681
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20595}

tools_webrtc/libs/generate_licenses.py

@@ -23,9 +23,11 @@ import subprocess
 
 LIB_TO_LICENSES_DICT = {
     'android_tools': ['third_party/android_tools/LICENSE'],
+    'auto': ['third_party/auto/src/LICENSE.txt'],
     'boringssl': ['third_party/boringssl/src/LICENSE'],
     'errorprone': ['third_party/errorprone/LICENSE'],
     'expat': ['third_party/expat/files/COPYING'],
+    'guava': ['third_party/guava/LICENSE'],
     'ijar': ['third_party/ijar/LICENSE'],
     'jsoncpp': ['third_party/jsoncpp/LICENSE'],
     'libc++': ['buildtools/third_party/libc++/trunk/LICENSE.TXT'],