Add internals API for highlighting text markers and use it for SVG tests

This patch adds a new API to internals for highlighting text match
markers: setMarkedTextMatchesAreHighlighted. This turns on the
highlights on document markers which are normally enabled when searching
for a string via ctrl+f.

As a followup to r166368[1], I've converted an existing manual test
to an automated test.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=166368

BUG=56580

Review URL: https://codereview.chromium.org/154533002

git-svn-id: svn://svn.chromium.org/blink/trunk@166549 bbb929c8-8fbe-4397-9dbb-9b2b20218538