third_party/.gitignore: Add /androidx/lib/

Add a missing entry to this .gitignore file to avoid
'git status' displaying that directory as untracked,
which is super annoying.

BUG=NONE
R=digit@chromium.org
TBR=thakis@chromium.org

Change-Id: I1e221e6bb5e3fb76bd1c0fef9510f88c00414c46
Reviewed-on: https://chromium-review.googlesource.com/c/1449654Reviewed-by: David Turner <digit@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Auto-Submit: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628300}

third_party/.gitignore

@@ -21,6 +21,7 @@
 /android_tools/
 /android_tools_internal/
 /android_webview_glue/src
+/androidx/lib/
 /angle
 /angle_dx11
 /apache-portable-runtime/src