Refactor and comment BreakingContext::handleText()

Refactor and comment BreakingContext::handleText()

This is a large loop condition which spends a lot of its time working out
whether the current whitespace character is breakable or not. Refactor it
to make its operation clearer and add some comments that make it easier
to take in at a glance what the loop is doing.

This first part of the refactoring gets rid of a >100 line if-clause and converts
it to an early |continue|. There is just no way of making this part of the
refactoring easy to follow. :) (It's even worse if I duplicate the block of code
I've here put into a prepareNewCharacter() function.)

This is the first of a number of planned passes to make this function more friendly.

BUG=567080
Committed: https://crrev.com/65ecdc54baece8049b76ee11a1ed69f9436d5be9
Cr-Commit-Position: refs/heads/master@{#365629}

patch from issue 1504083002 at patchset 60001 (http://crrev.com/1504083002#ps60001)