WebKitCSSMatrix: Avoid crash resolving relative lengths.

WebKitCSSMatrix: Avoid crash resolving relative lengths.

The initial styles created to resolve lengths did not properly construct
a font which caused a crash trying to resolve 'ex' and 'ch' units. Call
Font::update() to construct a dummy font.

There is no document here to the deduce the initial font from. What to do
with relative lengths and percentages has been discussed for the
standardization of DOMMatrix, and the current draft says only absolute
lengths should be allowed [1].

I chose not to try to align with that as WebKitCSSMatrix is a proprietary
api. FWIW, viewport relative units were already resolved against an emtpy
viewport.

[1] http://dev.w3.org/fxtf/geometry/#dommatrix-constructors

R=leviw@chromium.org
BUG=414145
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=191865

[rune, 2015-03-12 15:22:10]

This is based on top of https://codereview.chromium.org/1001833002/

[rune, 2015-03-13 10:02:31]

rune@opera.com changed reviewers:
+ timloh@chromium.org

[rune, 2015-03-13 10:02:32]

I just realized timloh has looked at this before and actually reported the bug.

[leviw_travelin_and_unemployed, 2015-03-13 18:23:34]

LGTM.

[rune, 2015-03-13 18:29:58]

The CQ bit was checked by rune@opera.com

[commit-bot: I haz the power, 2015-03-13 18:30:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1005523002/1

[commit-bot: I haz the power, 2015-03-13 20:03:08]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=191865