Fix incorrect number of matches and misleading highlighting in "Find on Page" results.

Now search will continue from the end position of the last successful search 
instead of position next to the start of the last search.
After this fix TextFinder shouldn't  be able to find overlapping matches.

TEST=webkit_unit_tests --gtest_filter=TextFinder*.*
BUG=397563

Review URL: https://codereview.chromium.org/429193002

git-svn-id: svn://svn.chromium.org/blink/trunk@180767 bbb929c8-8fbe-4397-9dbb-9b2b20218538