Include previous invalidation rects in GraphicsLayerDebugInfo json

This is a follow up for http://crbug.com/617723002 .
This CL changes GraphicsLayerDebugInfo json to include the previous invalidation rects.

FrameViewer displays invalidation rects in compositing TraceEvent,
which is after picture recording in GraphicsLayer::paintGraphicsLayerContents.
Before this change, the invalidation rects were always cleared at
GraphicsLayer::paintGraphicsLayerContents, displaying nothing.
This CL fixes it by returning the invalidation rects used in recording just before
the compositing event.

BUG=402033

Review URL: https://codereview.chromium.org/615223004

git-svn-id: svn://svn.chromium.org/blink/trunk@183176 bbb929c8-8fbe-4397-9dbb-9b2b20218538