AW docs: expand on commandline docs

No change to logic, only docs.

This expands on commandline docs for WebView, to clarify that all
WebView providers support commandline flags in the same way, and to
advise how to properly use tooling (since some tools treat Monochrome
more like Chrome than like WebView).

This has a minor modification to remove --log-net-log as an example
flag, because we prefer users follow
https://chromium.googlesource.com/chromium/src/+/HEAD/android_webview/docs/net-debugging.md
instead of applying this flag themselves.

This is inspired by the comment raised in https://crbug.com/977784#c19.

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I090c27082d072b8ad434702e19efea8d525a6aa8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674445
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671787}

android_webview/docs/commandline-flags.md

@@ -8,43 +8,68 @@ check with `adb shell getprop ro.build.type`). Flags cannot be enabled on
 production builds of Android.
 ***
 
-WebView reads flags from a specific file during startup. To enable flags, write
-to the file with:
+WebView reads flags from a specific file on the device as part of the startup
+sequence. Therefore, it's important to always **kill the WebView-based app**
+you're examining after modifying commandline flags, to ensure the flags are
+picked up during the next app restart.
+
+WebView always looks for the same file on the device
+(`/data/local/tmp/webview-command-line`), regardless of which package is the
+[the WebView provider](prerelease.md).
+
+### Python script
+
+The simplest way to set WebView flags is with the dedicated python script. This
+works regardless of which package is the WebView provider:
 
 ```sh
 # Overwrite flags (supports multiple)
-build/android/adb_system_webview_command_line \
-    --show-composited-layer-borders \
-    --log-net-log=foo.json
+build/android/adb_system_webview_command_line --show-composited-layer-borders --webview-log-js-console-messages
 # Clear flags
 build/android/adb_system_webview_command_line ""
 # Print flags
 build/android/adb_system_webview_command_line
 ```
 
-Or, for a locally compiled APK:
+### Generated Wrapper Script
+
+If you have a locally compiled APK, you may instead set flags using the
+Generated Wrapper Script like so:
 
 ```sh
 autoninja -C out/Default system_webview_apk
 # Overwrite flags (supports multiple)
-out/Default/bin/system_webview_apk argv --args='--show-composited-layer-borders --log-net-log=foo.json'
+out/Default/bin/system_webview_apk argv --args='--show-composited-layer-borders --webview-log-js-console-messages'
 # Clear flags
 out/Default/bin/system_webview_apk argv --args=''
 # Print flags
 out/Default/bin/system_webview_apk argv
 ```
 
+*** note
+**Note:** be careful if using a `monochrome_*` target, as the Generated Wrapper
+Script writes to Chrome browser's flags file, and WebView **will not pick up
+these flags**. If using Monochrome, you can set flags with the
+`system_webview_*` Generated Wrapper Scripts, or use one of the other methods
+in this doc.
+***
+
+### Manual
+
 Or, you can use the `adb` in your `$PATH` like so:
 
 ```sh
 FLAG_FILE=/data/local/tmp/webview-command-line
-# Overwrite flags
-adb shell "echo '_ --show-composited-layer-borders' > ${FLAG_FILE}"
-# The first token is ignored. We use '_' as a convenient placeholder, but any
-# token is acceptable.
+# Overwrite flags (supports multiple). The first token is ignored. We use '_'
+# as a convenient placeholder, but any token is acceptable.
+adb shell "echo '_ --show-composited-layer-borders --webview-log-js-console-messages' > ${FLAG_FILE}"
+# Clear flags
+adb shell "rm ${FLAG_FILE}"
+# Print flags
+adb shell "cat ${FLAG_FILE}"
 ```
 
-### Applying Features with flags
+## Applying Features with flags
 
 WebView supports the same `--enable-features=feature1,feature2` and
 `--disable-features=feature3,feature4` syntax as the rest of Chromium. You can
@@ -59,7 +84,7 @@ Some interesting flags and Features:
 
  * `--show-composited-layer-borders`
  * `--enable-features=NetworkService,NetworkServiceInProcess`
- * `--log-net-log=<filename.json>`
+ * `--webview-log-js-console-messages`
 
 WebView also defines its own flags and Features: