Added a mock gesture tap highlight to allow reftesting layout tests.

Until now, the gesture tap highlight layout tests used reftests to match
expectations since the RenderBox and LinkHighlights produced pixel-identical
results. However, the underlying code paths to draw the rounded rects use
different methods (see bug) and a change in Skia has broken these tests.

This CL adds a setting to Blink to draw mock LinkHighlights that are
rectangular. This setting gets enabled only in tests and the expectation
files now just draw an ordinary rectangular div. We still have coverage for
rounding in the remaining pixel tests.

BUG=399719

Review URL: https://codereview.chromium.org/444083002

git-svn-id: svn://svn.chromium.org/blink/trunk@179673 bbb929c8-8fbe-4397-9dbb-9b2b20218538