Add setting to blend background with existing content

If both base background color (WebView::setBaseBackgroundColor) and
background color (set by document) has alpha, then the background color
should be blended into existing content.

This behavior is important in Android WebView, which is a view inside the
Android view tree and needs to blend with views below it.

Internal bug: 10750978

BUG=

Review URL: https://codereview.chromium.org/23483051

git-svn-id: svn://svn.chromium.org/blink/trunk@163657 bbb929c8-8fbe-4397-9dbb-9b2b20218538