Isolate elements are treated as neutral in UBA however Blink UBA implementation iterates over all the isolate characters.
The purpose of this patch is reduce the algorithm complexity skipping isolate elements when found in the UBA. 
Additionaly, rename the pastEnd variable to lastParagraphEnded.

BUG=

Review URL: https://chromiumcodereview.appspot.com/24055009

git-svn-id: svn://svn.chromium.org/blink/trunk@157874 bbb929c8-8fbe-4397-9dbb-9b2b20218538