Plugin Power Saver: Fix poster-click flaky test.

components/plugins/renderer/loadable_plugin_placeholder.cc

Index: components/plugins/renderer/loadable_plugin_placeholder.cc
diff --git a/components/plugins/renderer/loadable_plugin_placeholder.cc b/components/plugins/renderer/loadable_plugin_placeholder.cc
index 8bb9d4ad28c264a74fdd314c32694b839b04689b..90c3f9ba7183580e8f0c07d6736c85feda4421e8 100644
--- a/components/plugins/renderer/loadable_plugin_placeholder.cc
+++ b/components/plugins/renderer/loadable_plugin_placeholder.cc
@@ -6,20 +6,27 @@
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
+#include "base/command_line.h"
 #include "base/json/string_escape.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/values.h"
+#include "content/public/child/v8_value_converter.h"
+#include "content/public/common/content_switches.h"
 #include "content/public/renderer/render_frame.h"
 #include "content/public/renderer/render_thread.h"
 #include "gin/handle.h"
 #include "gin/object_template_builder.h"
+#include "third_party/WebKit/public/web/WebDOMMessageEvent.h"
+#include "third_party/WebKit/public/web/WebDocument.h"
 #include "third_party/WebKit/public/web/WebElement.h"
 #include "third_party/WebKit/public/web/WebInputEvent.h"
 #include "third_party/WebKit/public/web/WebKit.h"
 #include "third_party/WebKit/public/web/WebLocalFrame.h"
 #include "third_party/WebKit/public/web/WebPluginContainer.h"
 #include "third_party/WebKit/public/web/WebScriptSource.h"
+#include "third_party/WebKit/public/web/WebSerializedScriptValue.h"
 #include "third_party/WebKit/public/web/WebView.h"
 #include "third_party/re2/re2/re2.h"
 
@@ -331,6 +338,32 @@ void LoadablePluginPlaceholder::DidFinishLoadingCallback() {
   // This is necessary to prevent a flicker.
   if (premade_throttler_ && power_saver_enabled_)
     premade_throttler_->SetHiddenForPlaceholder(true /* hidden */);
+
+  // Set an attribute and post an event, so browser tests can wait for the
+  // placeholder to be ready to receive simulated user input.
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kEnablePluginPlaceholderTesting)) {
+    WebElement element = plugin()->container()->element();
+    element.setAttribute("placeholderLoaded", "true");
+
+    scoped_ptr<content::V8ValueConverter> converter(
+        content::V8ValueConverter::create());
+    base::StringValue value("placeholderLoaded");
+    blink::WebSerializedScriptValue message_data =
+        blink::WebSerializedScriptValue::serialize(converter->ToV8Value(
+            &value, element.document().frame()->mainWorldScriptContext()));
+
+    blink::WebDOMEvent event = element.document().createEvent("MessageEvent");
+    blink::WebDOMMessageEvent msg_event = event.to<blink::WebDOMMessageEvent>();
+    msg_event.initMessageEvent("message",     // type
+                               false,         // canBubble
+                               false,         // cancelable
+                               message_data,  // data
+                               "",            // origin [*]
+                               NULL,          // source [*]
+                               "");           // lastEventId
+    element.dispatchEvent(msg_event);
+  }
 }
 
 void LoadablePluginPlaceholder::SetPluginInfo(