Disentangle filter content recording state from FilterData

Disentangle filter content recording state from FilterData

In circumstances where an image is being painted, and said image is part
of the source graphics for a filter - and the image is animated and
needs to catch up - the FilterData for the filter can be destroyed while
the source graphic is being recorded.
While arguably this shouldn't happen at all, make sure that at least the
GC we're currently painting on isn't destroyed by moving it to a
separate structure housed by SVGPaintContext.

BUG=505444
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198266

[fs, 2015-07-02 14:10:18]

fs@opera.com changed reviewers:
+ pdr@chromium.org, schenney@chromium.org

[fs, 2015-07-02 15:12:07]

Maybe a tad ugly - could probably use some additional code shuffle - but that
could be improved upon in iterations. Doesn't fix the imageChanged invocation as
such, but moves us back to where we used to be wrt GraphicsContext lifetimes.
(It'd be nice to never have imageChanged during paint - maybe have animation
updates tied to "rAF".)

[Stephen Chennney, 2015-07-02 15:55:14]

LGTM

[fs, 2015-07-03 07:57:57]

The CQ bit was checked by fs@opera.com

[commit-bot: I haz the power, 2015-07-03 07:58:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1220053003/60001

[commit-bot: I haz the power, 2015-07-03 08:02:29]

Committed patchset #4 (id:60001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198266