AW docs: add net debugging to navbar.md

net-debugging.md was accidentally omitted from navbar.md, making it less
discoverable. This adds it in, so this page can be discovered from the
grey-ish top bar of the other docs pages.

Details on navbar can be found:
https://gerrit.googlesource.com/gitiles/+/HEAD/Documentation/markdown.md#Navigation-bar

R=ctzsm@chromium.org

Bug: 918221
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ia3e870833a3e86c8703301e296effce7139e66dd
Reviewed-on: https://chromium-review.googlesource.com/c/1488533
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635434}

android_webview/docs/navbar.md

@@ -3,5 +3,6 @@
 * [Home][home]
 * [Test instructions](/android_webview/docs/test-instructions.md)
 * [Commandline flags](/android_webview/docs/commandline-flags.md)
+* [Net debugging](/android_webview/docs/net-debugging.md)
 
 [home]: /android_webview/docs/README.md