Fix find on web pages with -webkit-user-select: none

Fix find on web pages with -webkit-user-select: none

[Title] Fix find on web pages with -webkit-user-select: none
[Issue#] https://tizendev.org/bugs/browse/N_SE-25811
[Problem] Focus does not change to the matched string on web pages with -webkit-user-select: none
[Cause] frame selection returns NULL on web pages with -webkit-user-select: none and
        current WebKit2 find string algorithm is using the frame selection.
[Solution] Highlight the active matching string using the document's active marker function
           instead of frame selection. For this operation, findString() explictly index
           the current active matching string and scroll to the position.

Change-Id: I498c944c34be3b2c7bd76cd5ad506f7d11cc9b9e