AW docs: clarify netlog and commandline flag requirements

No change to logic, only docs.

This clarifies that WebView requires debuggable Android images for all
commandline flags, including netlog. This repeats a code snippet for
checking if your device supports commandline flags.

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I03ebc095ee78744e9dfdbf9c113b418c57a53e7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2029190
Reviewed-by: Shimi Zhang <[email protected]>
Commit-Queue: Nate Fischer <[email protected]>
Cr-Commit-Position: refs/heads/master@{#736640}
3 files changed