Windows: Improvements to off-screen rendering support (issue #518).

- Implement support for transparency. Change the backing store to use Skia instead of GDI and to keep alpha values.
- Implement support for select popups. Requires a patch to Chromium (content_popups.patch).
- Implicitly disable accelerated compositing when using off-screen rendering.
- Introduce a new CefMouseEvent structure for representing mouse event information passed to CefBrowser methods.
- Merge cef_key_event_modifiers_t into cef_event_flags_t.
- Add a new argument to CefBrowser::Invalidate telling the browser whether to invalidate the select popup or the main view.
- Add a new cefclient "transparent-painting-enabled" command-line flag to test transparent off-screen rendering.
- Add a new cefclient "Transparency" test.
- Fix the cefclient off-screen rendering rotation effect to work even when the underlying web content is not updating.
- Add unit tests for transparent painting and select popups.

git-svn-id: https://chromiumembedded.googlecode.com/svn/trunk@979 5089003a-bbd8-11dd-ad1f-f1f9622dbc98

patch/patches/content_popups.patch

@@ -0,0 +1,31 @@
+Index: browser/web_contents/web_contents_impl.cc
+===================================================================
+--- browser/web_contents/web_contents_impl.cc	(revision 170167)
++++ browser/web_contents/web_contents_impl.cc	(working copy)
+@@ -1402,8 +1402,10 @@
+       new RenderWidgetHostImpl(this, process, route_id);
+   created_widgets_.insert(widget_host);
+ 
+-  RenderWidgetHostViewPort* widget_view =
+-      RenderWidgetHostViewPort::CreateViewForWidget(widget_host);
++  RenderWidgetHostViewPort* widget_view = RenderWidgetHostViewPort::FromRWHV(
++      view_->CreateViewForPopupWidget(widget_host));
++  if (!widget_view)
++    widget_view = RenderWidgetHostViewPort::CreateViewForWidget(widget_host);
+   if (!is_fullscreen) {
+     // Popups should not get activated.
+     widget_view->SetPopupType(popup_type);
+Index: public/browser/web_contents_view.h
+===================================================================
+--- public/browser/web_contents_view.h	(revision 170167)
++++ public/browser/web_contents_view.h	(working copy)
+@@ -37,6 +37,9 @@
+   virtual RenderWidgetHostView* CreateViewForWidget(
+       RenderWidgetHost* render_widget_host) = 0;
+ 
++  virtual RenderWidgetHostView* CreateViewForPopupWidget(
++      RenderWidgetHost* render_widget_host) { return NULL; }
++
+   // This is required because some WebContentsView cache the
+   // RenderWidgetHostView created by the above method. In that case, when the
+   // view created by the above method is destroyed, the old one needs to be set