AW docs: document support for CLI flags

This publicly documents WebView's long-standing support for commandline
flags, detailing any caveats.

R=ctzsm@chromium.org, tobiasjs@chromium.org

Bug: 918221
Test: None
Change-Id: I43f50ea7d481bd7b578b08f03dcb5737cc786469
Reviewed-on: https://chromium-review.googlesource.com/c/1392364
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619766}

android_webview/docs/commandline-flags.md

+# Commandline flags
+
+## Applying flags
+
+**Note:** this requires either a `userdebug` or `eng` Android build (you can
+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:
+
+```sh
+$ # Overwrites all flags (and prints the new flag state):
+$ build/android/adb_system_webview_command_line \
+    --show-composited-layer-borders \
+    --log-net-log=foo.json # Supports multiple flags
+$ # Simply prints the existing flag state):
+$ build/android/adb_system_webview_command_line
+$ # Passing empty string clears all flags:
+$ build/android/adb_system_webview_command_line ""
+```
+
+Or, you can use the `adb` in your `$PATH` like so:
+
+```sh
+$ FLAG_FILE=/data/local/tmp/webview-command-line
+$ 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.
+```
+
+**Note:** either set of commands will overwrite existing 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
+use these like any other flag. Please consult
+[`base/feature_list.h`](https://cs.chromium.org/chromium/src/base/feature_list.h)
+for details.
+
+## Interesting flags
+
+WebView supports any flags supported in any layer we depend on (ex. content).
+Some interesting flags and Features:
+
+ * `--show-composited-layer-borders`
+ * `--enable-features=NetworkService,NetworkServiceInProcess`
+ * `--log-net-log=<filename.json>`
+
+WebView also defines its own flags and Features:
+
+ * [AwSwitches.java](https://cs.chromium.org/chromium/src/android_webview/java/src/org/chromium/android_webview/AwSwitches.java)
+   (and its [native
+   counterpart](https://cs.chromium.org/chromium/src/android_webview/common/aw_switches.h))
+ * [AwFeatureList.java](https://cs.chromium.org/chromium/src/android_webview/java/src/org/chromium/android_webview/AwFeatureList.java)
+   (and its [native
+   counterpart](https://cs.chromium.org/chromium/src/android_webview/browser/aw_feature_list.h))
+
+## Implementation
+
+See [CommandLineUtil.java](https://cs.chromium.org/chromium/src/android_webview/java/src/org/chromium/android_webview/command_line/CommandLineUtil.java).