Fix a crash when chooseDesktopMedia is called with no target tab

chrome/browser/extensions/api/desktop_capture/desktop_capture_api.cc

Index: chrome/browser/extensions/api/desktop_capture/desktop_capture_api.cc
diff --git a/chrome/browser/extensions/api/desktop_capture/desktop_capture_api.cc b/chrome/browser/extensions/api/desktop_capture/desktop_capture_api.cc
index b9362800c0e881fec90e1e8b2a5c137637378515..905a9623bd757ed5352231ddbf25a91229c65e27 100644
--- a/chrome/browser/extensions/api/desktop_capture/desktop_capture_api.cc
+++ b/chrome/browser/extensions/api/desktop_capture/desktop_capture_api.cc
@@ -37,6 +37,7 @@ const char kInvalidTabIdError[] = "Invalid tab specified.";
 const char kTabUrlChangedError[] = "URL for the specified tab has changed.";
 const char kTabUrlNotSecure[] =
     "URL scheme for the specified tab is not secure.";
+const char kNoTabsAvailableError[] = "No tabs available.";
 
 DesktopCaptureChooseDesktopMediaFunction::PickerFactory* g_picker_factory =
     NULL;
@@ -89,7 +90,8 @@ bool DesktopCaptureChooseDesktopMediaFunction::RunAsync() {
       render_view_host()->GetProcess()->GetID(), request_id_, this);
 
   gfx::NativeWindow parent_window;
-  content::RenderViewHost* render_view;
+  content::RenderViewHost* render_view = NULL;
+  content::WebContents* web_contents = NULL;
   base::string16 target_name;
   if (params->target_tab) {
     if (!params->target_tab->url) {
@@ -112,7 +114,6 @@ bool DesktopCaptureChooseDesktopMediaFunction::RunAsync() {
       return false;
     }
 
-    content::WebContents* web_contents = NULL;
     if (!ExtensionTabUtil::GetTabById(*(params->target_tab->id), GetProfile(),
                                       true, NULL, NULL, &web_contents, NULL)) {
       error_ = kInvalidTabIdError;
@@ -135,9 +136,16 @@ bool DesktopCaptureChooseDesktopMediaFunction::RunAsync() {
     origin_ = GetExtension()->url();
     target_name = base::UTF8ToUTF16(GetExtension()->name());
     render_view = render_view_host();
-    parent_window =
-        GetAssociatedWebContents()->GetTopLevelNativeWindow();
+
+    web_contents = GetAssociatedWebContents();
+    if (!web_contents) {
+      error_ = kNoTabsAvailableError;

[zel, 2014/05/28 04:06:34]

I still think we could support this scenario as well if the UI displays system
modal dialog in the case when parent_window and web_contents are NULL. I am not
sure that we have real-life use case of that tough.

[Sergey Ulanov, 2014/06/01 01:31:38]

I agree. I think in this case we should pass null parent_web_contents to
DesktopMediaPickerDialog and then it should use
DialogDelegate::CreateDialogWidget() to create dialog instead of
WebContentsModalDialogManager to show the dialog (the way it worked before
https://codereview.chromium.org/281383008/diff/90001/chrome/browser/ui/views/...).

[tbarzic, 2014/06/03 01:18:51]

We still need to select the parent_window to use if GetAssociatedWebContents
returns NULL. Looking at the code, views seem to handle the situation where
parent_window is NULL well. I'm not sure what should be done on Mac, though

+      return false;
+    }
+
+    parent_window = web_contents->GetTopLevelNativeWindow();
   }
+
   render_process_id_ = render_view->GetProcess()->GetID();
   render_view_id_ = render_view->GetRoutingID();
 
@@ -208,7 +216,7 @@ bool DesktopCaptureChooseDesktopMediaFunction::RunAsync() {
   DesktopMediaPicker::DoneCallback callback = base::Bind(
       &DesktopCaptureChooseDesktopMediaFunction::OnPickerDialogResults, this);
 
-  picker_->Show(web_contents(),
+  picker_->Show(web_contents,
                 parent_window, parent_window,
                 base::UTF8ToUTF16(GetExtension()->name()),
                 target_name,