Pass GraphicsLayer to InspectorInstrumentation::didPaint(), include layer id in Paint event

We used to infer graphics layer ID from CompositedLayerMapping owner,
which only works properly for main graphics layer.
Moving instrumentation to CompositedLayerMapping::paintContents() lets
us capture correct layer id and also receive paint events from scrollbar
layers.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/59443003

git-svn-id: svn://svn.chromium.org/blink/trunk@161904 bbb929c8-8fbe-4397-9dbb-9b2b20218538