Allow to have a transparent UA dependent background.

chrome/browser/ui/views/frame/browser_view.cc

Index: chrome/browser/ui/views/frame/browser_view.cc
diff --git a/chrome/browser/ui/views/frame/browser_view.cc b/chrome/browser/ui/views/frame/browser_view.cc
index 2edae31c9bd16dcf71b9fbeb9b5191e72fbfeb29..89fe983205c81d3e52033f6fcb4b9ce4fddae398 100644
--- a/chrome/browser/ui/views/frame/browser_view.cc
+++ b/chrome/browser/ui/views/frame/browser_view.cc
@@ -2128,8 +2128,12 @@ void BrowserView::InitViews() {
   devtools_web_view_->SetVisible(false);
 
   contents_container_ = new views::View();
+  // FIXME: Clear alpha only upon a call to
+  // content::RenderWidgetHostView->SetBackgroundColor(SK_ColorTRANSPARENT).
   contents_container_->set_background(views::Background::CreateSolidBackground(
-      GetThemeProvider()->GetColor(ThemeProperties::COLOR_CONTROL_BACKGROUND)));
+      GetThemeProvider()->GetColor(ThemeProperties::COLOR_CONTROL_BACKGROUND) &
+      0x00FFFFFF));

[trchen, 2016/02/25 02:30:12]

What does this mean? Is it pre-multiplied alpha or not?
If pre-multiplied, masking alpha to 0 will result in invalid color.
If not pre-multiplied, masking alpha to 0 is equivalent to fully transparent.
Either way this looks weird and deserves a comment.

[Julien Isorce Samsung, 2016/02/25 11:33:55]

Sorry for the confusion, this code has no intention to be merged. In the first
comment of this CL I refer to that as "demo" code. I just force the theme to
have its background fully transparent.
In real world, the hypothetic custom theme would defines its background as fully
transparent.

+
   contents_container_->AddChildView(devtools_web_view_);
   contents_container_->AddChildView(contents_web_view_);
   contents_container_->SetLayoutManager(new ContentsLayoutManager(