Extract PaintInvalidationReason from RenderObject

Extract InvalidationReason from RenderObject, rename it to
PaintInvalidationReason and put it into
platform/graphics/PaintInvalidationReason.h so that we can pass
PaintInvalidationReason instead of const char* reason and delay
paintInvalidationReasonToString() when we really need the string.

BUG=410465

Review URL: https://codereview.chromium.org/638203002

git-svn-id: svn://svn.chromium.org/blink/trunk@183494 bbb929c8-8fbe-4397-9dbb-9b2b20218538