Do not merge DocumentMarkers of type TextMatch.

Markers added to a DocumentMarkerController get merged into a single marker when they overlap or are adjacent to each other. This causes problems for text match markers which can be individually activated, e.g. for stepping through results of a FindInPage operation. This patch fixes this problem by never merging DocumentMarkers of type TextMatch in DocumentMarkerController::addMarker().

BUG=142071

Review URL: https://chromiumcodereview.appspot.com/21372003

git-svn-id: svn://svn.chromium.org/blink/trunk@156293 bbb929c8-8fbe-4397-9dbb-9b2b20218538