Support transparent window in Views framework (follow up issue #2315)

Hi Team,

What I need is very similar to the issue #2315. I would like to provide my changes for the case.

First of all, the requirement I have is to create a non-rectangular window. One solution is implementing OSR to have parent window with transparent background. Another solution is creating a translucent window via Views frameworks, which is what this PR did.

The major changes are:

1. Configure opacity to views::Widget::InitParams::TRANSLUCENT_WINDOW while initializing the Widget controlled by CefWindowDelegate::IsTranslucent()

2. Set the background color of CefWindow to transparent if above is configured

3. Configure CefSettings.transparent_painting to true. It controls accepting alpha component while configuring the background in CefContext::GetBackgroundColor(). It is required for deciding the default background of WebView in renderer.

CefSettings.windowless_rendering_enabled is renamed to CefSettings.transparent_painting because I consider the flag only controlling the background color calculation logic.

I created a testing page using the Views framework as below:

The page can be launched by

Thank you for any feedback/comments in advance.