Make compositing/video/video-poster.html test dump the layer tree.

Instead of testing if the video was promoted to a composited layer by
checking if compositing mode is enabled (this is not a valid check
under ForceCompositingMode), dump the composited layer tree so that
the promotion or not is explicitly visible.

R=enne@chromium.org
BUG=361729

Review URL: https://codereview.chromium.org/228783006

git-svn-id: svn://svn.chromium.org/blink/trunk@171230 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[SVG Fonts] Fix <font-face> element leak document

Reported by hajimehoshi@: A svg document with <font-face> was never destructed from a reference cycle.

This patch breaks the reference cycle by making the reference from SVGFontFaceSource to SVGFontFaceElement weak.
SVGFontFaceSource only exist for in-document font face reference, so this change is safe.

This patch also validates SVGFontData's weak-ref to SVGFontFaceElement. SVGFontData should be only used while the document is alive. This CL changes the weak-ref to WeakPtr and ensure the svg tree is alive before reference.

TEST=LayoutTests/svg/svg-font-face-leak-document.html
BUG=270000

Review URL: https://codereview.chromium.org/216563002

git-svn-id: svn://svn.chromium.org/blink/trunk@171231 bbb929c8-8fbe-4397-9dbb-9b2b20218538