Added setters and getters in WebLayer.h / GraphicsLayer.h for:
* blendMode - specifying the formula for blending the layer with its backdrop - http://dev.w3.org/fxtf/compositing-1/#blending
* isIsolatedGroupRoot - this layer should make sure that all its blending child layers in the layer tree and the layer itself will use a transparent black initial backdrop. http://dev.w3.org/fxtf/compositing-1/#isolatedgroups

Added WebBlendMode.h defining the WebBlendMode enum and convertion functions.

Added a new compositing reason - CompositingReasonIsolateCompositedDescendants. A RenderLayer might require compositing in order to perform isolation of its composited blending children

BUG=243223

Review URL: https://codereview.chromium.org/23511004

git-svn-id: svn://svn.chromium.org/blink/trunk@163079 bbb929c8-8fbe-4397-9dbb-9b2b20218538