dragend fix for mac <webview>.

dragend would never fire on mac because
BrowserPluginEmbedder::SystemDragEnded() clears |guest_dragging_over_|, which stops
BrowserPluginEmbedder::DragSourceEndedAt() to send the dragend
message.
This isn't an issue in aura b/c, we see DrageSourceEndedAt() before
SystemDragEnded().

BUG=401941
Test=On mac, load a <webview> in a chrome app, register a "dragend"
listener to a draggable div. Drag that div, observe "dragend" listener
firing properly.

Review URL: https://codereview.chromium.org/499493002

Cr-Commit-Position: refs/heads/master@{#291356}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291356 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/browser_plugin/browser_plugin_embedder.cc

@@ -28,6 +28,7 @@ namespace content {
 
 BrowserPluginEmbedder::BrowserPluginEmbedder(WebContentsImpl* web_contents)
     : WebContentsObserver(web_contents),
+      guest_drag_ending_(false),
       weak_ptr_factory_(this) {
 }
 
@@ -54,6 +55,7 @@ void BrowserPluginEmbedder::DragLeftGuest(BrowserPluginGuest* guest) {
 
 void BrowserPluginEmbedder::StartDrag(BrowserPluginGuest* guest) {
   guest_started_drag_ = guest->AsWeakPtr();
+  guest_drag_ending_ = false;
 }
 
 WebContentsImpl* BrowserPluginEmbedder::GetWebContents() const {
@@ -65,6 +67,20 @@ BrowserPluginEmbedder::GetBrowserPluginGuestManager() const {
   return GetWebContents()->GetBrowserContext()->GetGuestManager();
 }
 
+void BrowserPluginEmbedder::ClearGuestDragStateIfApplicable() {
+  // The order at which we observe SystemDragEnded() and DragSourceEndedAt() is
+  // platform dependent.
+  // In OSX, we see SystemDragEnded() first, where in aura, we see
+  // DragSourceEndedAt() first. For this reason, we check if both methods were
+  // called before resetting |guest_started_drag_|.
+  if (guest_drag_ending_) {
+    if (guest_started_drag_)
+      guest_started_drag_.reset();
+  } else {
+    guest_drag_ending_ = true;
+  }
+}
+
 bool BrowserPluginEmbedder::DidSendScreenRectsCallback(
    WebContents* guest_web_contents) {
   static_cast<RenderViewHostImpl*>(
@@ -93,22 +109,23 @@ bool BrowserPluginEmbedder::OnMessageReceived(const IPC::Message& message) {
 
 void BrowserPluginEmbedder::DragSourceEndedAt(int client_x, int client_y,
     int screen_x, int screen_y, blink::WebDragOperation operation) {
-  if (guest_started_drag_.get()) {
+  if (guest_started_drag_) {
     gfx::Point guest_offset =
         guest_started_drag_->GetScreenCoordinates(gfx::Point());
     guest_started_drag_->DragSourceEndedAt(client_x - guest_offset.x(),
         client_y - guest_offset.y(), screen_x, screen_y, operation);
   }
+  ClearGuestDragStateIfApplicable();
 }
 
 void BrowserPluginEmbedder::SystemDragEnded() {
   // When the embedder's drag/drop operation ends, we need to pass the message
   // to the guest that initiated the drag/drop operation. This will ensure that
   // the guest's RVH state is reset properly.
-  if (guest_started_drag_.get())
+  if (guest_started_drag_)
     guest_started_drag_->EndSystemDrag();
-  guest_started_drag_.reset();
   guest_dragging_over_.reset();
+  ClearGuestDragStateIfApplicable();
 }
 
 void BrowserPluginEmbedder::OnUpdateDragCursor(bool* handled) {

content/browser/browser_plugin/browser_plugin_embedder.h

@@ -75,6 +75,8 @@ class CONTENT_EXPORT BrowserPluginEmbedder : public WebContentsObserver {
 
   BrowserPluginGuestManager* GetBrowserPluginGuestManager() const;
 
+  void ClearGuestDragStateIfApplicable();
+
   bool DidSendScreenRectsCallback(WebContents* guest_web_contents);
 
   bool SetZoomLevelCallback(double level, WebContents* guest_web_contents);
@@ -106,6 +108,9 @@ class CONTENT_EXPORT BrowserPluginEmbedder : public WebContentsObserver {
   // status messages to the correct guest.
   base::WeakPtr<BrowserPluginGuest> guest_started_drag_;
 
+  // Keeps track of "dragend" state.
+  bool guest_drag_ending_;
+
   base::WeakPtrFactory<BrowserPluginEmbedder> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserPluginEmbedder);