Add a basic postMessage test for <webview> in a new test suite.

chrome/browser/apps/guest_view/web_view_browsertest.cc

Index: chrome/browser/apps/guest_view/web_view_browsertest.cc
diff --git a/chrome/browser/apps/guest_view/web_view_browsertest.cc b/chrome/browser/apps/guest_view/web_view_browsertest.cc
index 39203974b6caf49c2fdfa5ac8601460d4813de67..1947ad985fc6a540c235c807abe5692407bc5524 100644
--- a/chrome/browser/apps/guest_view/web_view_browsertest.cc
+++ b/chrome/browser/apps/guest_view/web_view_browsertest.cc
@@ -844,6 +844,10 @@ class WebViewTest : public extensions::PlatformAppBrowserTest {
   content::WebContents* embedder_web_contents_;
 };
 
+// Test suite that containts tests that are meant to run with and without
+// --site-per-process.
+class WebViewCommonTest : public extensions::PlatformAppBrowserTest {};
+
 class WebViewDPITest : public WebViewTest {
  protected:
   void SetUpCommandLine(base::CommandLine* command_line) override {
@@ -2711,3 +2715,9 @@ IN_PROC_BROWSER_TEST_F(WebViewTest, AllowTransparencyAndAllowScalingPropagate) {
   ASSERT_TRUE(guest->allow_transparency());
   ASSERT_TRUE(guest->allow_scaling());
 }
+
+IN_PROC_BROWSER_TEST_F(WebViewCommonTest, BasicPostMessage) {
+  ASSERT_TRUE(StartEmbeddedTestServer());  // For serving guest pages.
+  ASSERT_TRUE(RunPlatformAppTest("platform_apps/web_view/post_message/basic"))
+      << message_;
+}