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For example, android can set the background color of a WebView to be
transparent. This sets the mainframe base background color to be transparent.

The internal bug is b/15373310
BUG=375864

Review URL: https://codereview.chromium.org/315763003

git-svn-id: svn://svn.chromium.org/blink/trunk@175586 bbb929c8-8fbe-4397-9dbb-9b2b20218538