Show action mode bar on Android WebView Test Shell

Basically do what ContentViewClient.startActionMode does.

BUG=501072

Review URL: https://codereview.chromium.org/1193463003

Cr-Commit-Position: refs/heads/master@{#334732}

android_webview/test/shell/src/org/chromium/android_webview/shell/AwShellActivity.java

@@ -10,6 +10,7 @@
 import android.content.SharedPreferences;
 import android.os.Bundle;
 import android.text.TextUtils;
+import android.view.ActionMode;
 import android.view.Gravity;
 import android.view.KeyEvent;
 import android.view.View;
@@ -38,6 +39,9 @@
 import org.chromium.android_webview.test.AwTestContainerView;
 import org.chromium.android_webview.test.NullContentsClient;
 import org.chromium.base.CommandLine;
+import org.chromium.content.browser.SelectActionMode;
+import org.chromium.content.browser.SelectActionModeCallback;
+import org.chromium.content.browser.SelectActionModeCallback.ActionHandler;
 import org.chromium.content_public.browser.LoadUrlParams;
 import org.chromium.content_public.browser.NavigationController;
 import org.chromium.content_public.browser.WebContents;
@@ -144,6 +148,16 @@ public boolean shouldOverrideKeyEvent(KeyEvent event) {
                 }
                 return false;
             }
+
+            @Override
+            public SelectActionMode startActionMode(
+                    View view, ActionHandler actionHandler, boolean floating) {
+                if (floating) return null;
+                ActionMode.Callback callback =
+                        new SelectActionModeCallback(view.getContext(), actionHandler);
+                ActionMode actionMode = view.startActionMode(callback);
+                return actionMode != null ? new SelectActionMode(actionMode) : null;
+            }
         };
 
         SharedPreferences sharedPreferences =