Fix link highlights on links that contain images.

Link highlights are now generated by walking down the render tree and
accumulating quads, rather than taking one bounding quad.

The real issue was with RenderInlines; absoluteQuads would return the
quads for the line box but only as calculated from line-height and font
metrics. This meant that replaced elements like <img> would overflow out of
the highlight. With this approach we get a highlight that hopefully
matches the actual hit testing region.

BUG=289764

Review URL: https://codereview.chromium.org/129643002

git-svn-id: svn://svn.chromium.org/blink/trunk@165024 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make TreeScope::baseURL pure virtual

This allows completeURL and baseURL work correctly when called on a ShadowRoot.
They should fall back to the ShadowRoot's Document's baseURL unless the ShadowRoot
itself has a baseURL specified.

Since no code in Blink yet calls either on a ShadowRoot, there's no
test I can write that triggers this, but once we start calling
treeScope().completeURL() this will be well-tested.

R=esprehn@chromium.org
BUG=311682

Review URL: https://codereview.chromium.org/135643004

git-svn-id: svn://svn.chromium.org/blink/trunk@165025 bbb929c8-8fbe-4397-9dbb-9b2b20218538