Add pointer-events="bounding-box" for svg content.

The 'bounding-box' keyword in 'pointer-events' enables
svg authors to leave out certain invisible 'hit-area'
elements, and instead get them generated based on the
bounding box. This is convenient for example if one wants
multiline text in svg to respond to mouse events over the
entire region it covers. Another common use-case is
groups, which are container elements. These elements
currently are not possible to target directly, since
they only get events through bubbling from their
children.

This patch makes container elements targetable if one
specifies 'pointer-events: bounding-box' on them (either
as an svg presentation attribute or in css).

The 'bounding-box' keyword is specified in SVG2, 
https://svgwg.org/svg2-draft/interact.html#PointerEventsProp. 

Note: The keyword was called 'boundingbox' in SVG Tiny 1.2,
http://www.w3.org/TR/SVGTiny12/interact.html#PointerEventsProperty,
but it was renamed to be more in sync with CSS syntax in general.

R=pdr
BUG=

Review URL: https://codereview.chromium.org/45733010

git-svn-id: svn://svn.chromium.org/blink/trunk@163425 bbb929c8-8fbe-4397-9dbb-9b2b20218538