AW: Add a GN build flag to show a launcher icon for dev ui

* Add a GN argument to whether show a launcher icon to open WebView
  developer UI main. This gn argument is off by default.
* The shown icon will be another icon in addition to any icons added by
  WebView providers.
* A launcher activity is defined as an activity-alias that links to
  developer UI main activity.
* The manifest file where this activity-alias is defined is merged into
  WebView manifest only if the flag is true.

Bug: 989831
Test: Manually inspect the new added launcher icon after installing system_webview_apk, monochrome_apk, trichrome with gn arg 'webview_devui_show_icon=true`
Change-Id: I9e5b808f5e1f9491d2ed813ccf342207a7642b9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863929
Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Auto-Submit: Hazem Ashmawy <hazems@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711391}

android_webview/apk/BUILD.gn

@@ -106,6 +106,10 @@ android_resources("icon_resources") {
 android_resources("devui_resources") {
   resource_dirs = [ "java/res_devui" ]
   custom_package = "org.chromium.android_webview.devui"
+
+  if (webview_devui_show_icon) {
+    android_manifest = "java/DeveloperUiLauncherManifest.xml"
+  }
 }
 
 jinja_template("system_webview_manifest") {

android_webview/apk/java/DeveloperUiLauncherManifest.xml

+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  Copyright (c) 2019 The Chromium Authors. All rights reserved.  Use of this
+  source code is governed by a BSD-style license that can be found in the
+  LICENSE file.
+-->
+
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+          package="org.chromium.android_webview.devui">
+
+    <!--suppress HardcodedText -->
+    <application>
+        <activity-alias android:name="org.chromium.android_webview.devui.LauncherActivity"
+                  android:targetActivity="org.chromium.android_webview.devui.MainActivity"
+                  android:label="WebView DevTools"
+                  android:exported="true">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN"/>
+                <category android:name="android.intent.category.LAUNCHER"/>
+            </intent-filter>
+        </activity-alias>
+    </application>
+</manifest>

android_webview/variables.gni

@@ -2,6 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+declare_args() {
+  # Show a launcher icon to open WebView developer UI, off by default.
+  webview_devui_show_icon = false
+}
+
 system_webview_android_manifest =
     "$root_gen_dir/android_webview/system_webview_apk/AndroidManifest.xml"
 trichrome_webview_android_manifest =