Fix for MAINT-7787: [Alex Ivy] RC Viewers in 5.1.0 range show black background...

Fix for MAINT-7787: [Alex Ivy] RC Viewers in 5.1.0 range show black background for CONTENT_TYPE_HTML. (The color we pass to CEF has 4 compontents red, blue, green and alpha.  We were only passing 3 and setting the color and not the alpha. At somepoint in CEF's history the default value must have changed. Passing 0xFF for alpha too - i.e. fully opaque - fixes this)