In text iterator, the SearchBuffer is reset when the iterator passes through
a 0-length boundary, such as IMG element. This causes failure to find texts
containing parts from both before and after the image.

BUG=419524

Review URL: https://codereview.chromium.org/624483002

git-svn-id: svn://svn.chromium.org/blink/trunk@183345 bbb929c8-8fbe-4397-9dbb-9b2b20218538