DevTools: add image element reference and resource url to deferred ImageDecode events in Timeline

- track lfetime and usage of LazyDecodingPixelRef in TimelineTraceEventProcessor;
- keep node/url info from RenderImage being painted when lazy pixel ref is used to draw an image;
- use pixelRef from DecodeImageTask to associate deferred decode with node/url info;
- only emit timeline record when we actually perform image decode from within ImageDecodeTask.

Related chromium change: https://codereview.chromium.org/23537033
Also depends on https://codereview.chromium.org/23542019/
BUG=
R=hclam@chromium.org, pfeldman@chromium.org, senorblanco@chromium.org

Review URL: https://codereview.chromium.org/24096002

git-svn-id: svn://svn.chromium.org/blink/trunk@158321 bbb929c8-8fbe-4397-9dbb-9b2b20218538